Internet Engineering Task Force (IETF)                          C. Daboo
Request for Comments: 6638                                    Apple Inc.
Updates: 4791, 5546                                      B. Desruisseaux
Category: Standards Track                                         Oracle
ISSN: 2070-1721                                                June 2012

Scheduling Extensions to CalDAV

Abstract

   This document defines extensions to the Calendaring Extensions to
   WebDAV (CalDAV) "calendar-access" feature to specify a standard way
   of performing scheduling operations with iCalendar-based calendar
   components.  This document defines the "calendar-auto-schedule"
   feature of CalDAV.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6638.

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   1. Introduction ....................................................5
      1.1. Terminology ................................................6
      1.2. Notational Conventions .....................................7
      1.3. XML Namespaces and Processing ..............................7
   2. Scheduling Support ..............................................8
      2.1. Scheduling Outbox Collection ...............................9
           2.1.1. CALDAV:schedule-outbox-URL Property ................10
      2.2. Scheduling Inbox Collection ...............................10
           2.2.1. CALDAV:schedule-inbox-URL Property .................11
      2.3. Calendaring Reports Extensions ............................12
      2.4. Additional Principal Properties ...........................12
           2.4.1. CALDAV:calendar-user-address-set Property ..........12
           2.4.2. CALDAV:calendar-user-type Property .................13
   3. Scheduling Operations ..........................................14
      3.1. Identifying Scheduling Object Resources ...................14
      3.2. Handling Scheduling Object Resources ......................15
           3.2.1. Organizer Scheduling Object Resources ..............15
                  3.2.1.1. Create ....................................16
                  3.2.1.2. Modify ....................................17
                  3.2.1.3. Remove ....................................18
           3.2.2. Attendee Scheduling Object Resources ...............18
                  3.2.2.1. Allowed "Attendee" Changes ................18
                  3.2.2.2. Create ....................................19
                  3.2.2.3. Modify ....................................20
                  3.2.2.4. Remove ....................................21
           3.2.3. HTTP Methods .......................................21
                  3.2.3.1. PUT .......................................22
                  3.2.3.2. DELETE ....................................22
                  3.2.3.3. COPY ......................................23
                  3.2.3.4. MOVE ......................................24

           3.2.4. Additional Method Preconditions ....................24
                  3.2.4.1. CALDAV:unique-scheduling-object-resource
                           Precondition ..............................24
                  3.2.4.2. CALDAV:same-organizer-in-all-components
                           Precondition ..............................25
                  3.2.4.3. CALDAV:allowed-organizer-scheduling-
                              object-change Precondition .............25
                  3.2.4.4. CALDAV:allowed-attendee-scheduling-
                              object-change Precondition .............26
           3.2.5. DTSTAMP and SEQUENCE Properties ....................26
           3.2.6. Restrict Recurrence Instances Sent to "Attendees" ..27
           3.2.7. Forcing the Server to Send a Scheduling Message ....27
           3.2.8. "Attendee" Participation Status ....................28
           3.2.9. Schedule Status Values .............................29
           3.2.10. Avoiding Conflicts when Updating Scheduling Object
                   Resources .........................................31
                   3.2.10.1. PUT .....................................33
                   3.2.10.2. DELETE, COPY, or MOVE ...................33
   4. Processing Incoming Scheduling Messages ........................34
      4.1. Processing "Organizer" Requests, Additions, and
           Cancellations .............................................34
      4.2. Processing "Attendee" Replies .............................35
      4.3. Default Calendar Collection ...............................35
           4.3.1. Additional Method Preconditions ....................36
                  4.3.1.1. CALDAV:default-calendar-needed
                           Precondition ..............................36
                  4.3.1.2. CALDAV:valid-schedule-default-calendar-URL
                           Precondition ..............................36
   5. Request for Busy Time Information ..............................37
      5.1. Status Codes ..............................................38
      5.2. Additional Method Preconditions ...........................38
           5.2.1. CALDAV:valid-scheduling-message Precondition .......38
           5.2.2. CALDAV:valid-organizer Precondition ................39
   6. Scheduling Privileges ..........................................39
      6.1. Privileges on Scheduling Inbox Collections ................39
           6.1.1. CALDAV:schedule-deliver Privilege ..................40
           6.1.2. CALDAV:schedule-deliver-invite Privilege ...........40
           6.1.3. CALDAV:schedule-deliver-reply Privilege ............40
           6.1.4. CALDAV:schedule-query-freebusy Privilege ...........40
      6.2. Privileges on Scheduling Outbox Collections ...............40
           6.2.1. CALDAV:schedule-send Privilege .....................41
           6.2.2. CALDAV:schedule-send-invite Privilege ..............41
           6.2.3. CALDAV:schedule-send-reply Privilege ...............41
           6.2.4. CALDAV:schedule-send-freebusy Privilege ............41
      6.3. Aggregation of Scheduling Privileges ......................42

   7. Additional iCalendar Property Parameters .......................42
      7.1. Schedule Agent Parameter ..................................42
      7.2. Schedule Force Send Parameter .............................44
      7.3. Schedule Status Parameter .................................45
   8. Additional Message Header Fields ...............................46
      8.1. Schedule-Reply Request Header .............................46
      8.2. Schedule-Tag Response Header ..............................46
      8.3. If-Schedule-Tag-Match Request Header ......................47
   9. Additional WebDAV Properties ...................................47
      9.1. CALDAV:schedule-calendar-transp Property ..................47
      9.2. CALDAV:schedule-default-calendar-URL Property .............48
      9.3. CALDAV:schedule-tag Property ..............................49
   10. XML Element Definitions .......................................50
      10.1. CALDAV:schedule-response XML Element .....................50
      10.2. CALDAV:response XML Element ..............................50
      10.3. CALDAV:recipient XML Element .............................50
      10.4. CALDAV:request-status XML Element ........................51
   11. Security Considerations .......................................51
      11.1. Preventing Denial-of-Service Attacks .....................51
      11.2. Verifying Scheduling Operations ..........................52
      11.3. Verifying Busy Time Information Requests .................52
      11.4. Privacy Issues ...........................................53
      11.5. Mitigation of iTIP Threats ...............................53
   12. IANA Considerations ...........................................54
      12.1. Message Header Field Registrations .......................54
           12.1.1. Schedule-Reply ....................................54
           12.1.2. Schedule-Tag ......................................54
           12.1.3. If-Schedule-Tag-Match .............................54
      12.2. iCalendar Property Parameter Registrations ...............55
      12.3. iCalendar REQUEST-STATUS Value Registrations .............55
      12.4. Additional iCalendar Elements Registries .................55
           12.4.1. Schedule Agent Values Registry ....................56
           12.4.2. Schedule Force Send Values Registry ...............56
   13. Acknowledgements ..............................................56
   14. References ....................................................57
      14.1. Normative References .....................................57
      14.2. Informative References ...................................58

   Appendix A. Scheduling Privileges Summary .........................59
     A.1. Scheduling Inbox Privileges ................................59
     A.2. Scheduling Outbox Privileges ...............................60
   Appendix B. Example Scheduling Operations .........................60
     B.1. Example: "Organizer" Inviting Multiple "Attendees" .........61
     B.2. Example: "Attendee" Receiving an Invitation ................63
     B.3. Example: "Attendee" Replying to an Invitation ..............64
     B.4. Example: "Organizer" Receiving a Reply to an Invitation ....66
     B.5. Example: "Organizer" Requesting Busy Time Information ......69
     B.6. Example: User Attempting to Invite "Attendee" on
          Behalf of "Organizer" ......................................71
     B.7. Example: "Attendee" Declining an Instance of a
          Recurring Event ............................................72
     B.8. Example: "Attendee" Removing an Instance of a
          Recurring Event ............................................75

1. Introduction

   This document specifies extensions to the CalDAV "calendar-access"
   [RFC4791] feature to enable scheduling of iCalendar-based [RFC5545]
   calendar components between calendar users.

   This extension leverages the scheduling methods defined in the
   iCalendar Transport-independent Interoperability Protocol (iTIP)
   [RFC5546] to permit calendar users to perform scheduling operations
   such as schedule, reschedule, respond to scheduling request, or
   cancel calendar components, as well as search for busy time
   information.  However, the following iTIP [RFC5546] features are not
   covered: publishing, countering, delegating, refreshing, and
   forwarding calendar components, as well as replacing the "Organizer"
   of a calendar component.  It is expected that future extensions will
   be developed to address these.

   This specification defines a client/server scheduling protocol, where
   the server is made responsible for sending scheduling messages and
   processing incoming scheduling messages.  The client operations of
   creating, modifying, or deleting a calendar component in a calendar
   are enough to trigger the server to deliver the necessary scheduling
   messages to the appropriate calendar users.  This approach is
   sometimes referred to as "implicit scheduling".

   This specification only addresses how scheduling occurs with users on
   a single system (i.e., scheduling between CalDAV servers, or some
   other calendaring and scheduling system, is not defined).  However,
   this specification is compatible with servers being able to send or
   receive scheduling messages with "external" users (e.g., using the
   iCalendar Message-Based Interoperability Protocol (iMIP) [RFC6047]).

   Section 3 defines the automated "Scheduling Operations" that allow a
   client to store iCalendar data on a CalDAV server, with the server
   taking specific actions in response.  One of three scheduling
   operations can take place -- "create", "modify", or "remove", based
   on the HTTP method used for the request -- in addition to a
   comparison between any existing and any new iCalendar data.

   Section 4 defines how the server processes scheduling messages sent
   as the result of a scheduling operation.

   Section 5 defines how freebusy requests with an immediate response
   are accomplished.

   Section 6 defines access control privileges for the scheduling
   operations defined in this specification.

   For the majority of the following discussion, scheduling of events
   will be discussed.  However, scheduling of to-dos is also fully
   supported by this specification.

   This specification has been under development for a number of years,
   and most current implementations of CalDAV support it.  With the
   publication of this document, it is expected that all new CalDAV
   implementations will support it by default.  Interoperability tests
   have been performed regularly.  Significant issues with incompatible
   CalDAV implementations are not anticipated.

1.1. Terminology

   This specification reuses much of the same terminology as iCalendar
   [RFC5545], iTIP [RFC5546], WebDAV [RFC4918], and CalDAV [RFC4791].
   Additional terms used by this specification are as follows:

   Scheduling object resource:  A calendar object resource contained in
      a calendar collection for which the server will take care of
      sending scheduling messages on behalf of the owner of the calendar
      collection.

   Organizer scheduling object resource:  A scheduling object resource
      owned by the "Organizer".

   Attendee scheduling object resource:  A scheduling object resource
      owned by an "Attendee".

   Scheduling operation:  Add, change, or remove operations on a
      scheduling object resource for which the server will deliver
      scheduling messages to other calendar users.

   Scheduling message:  A calendar object that describes a scheduling
      operation such as schedule, reschedule, reply, or cancel.

   Scheduling Outbox collection:  A resource at which busy time
      information requests are targeted.

   Scheduling Inbox collection:  A collection in which incoming
      scheduling messages are delivered.

1.2. Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The Augmented BNF (ABNF) syntax used by this document to specify the
   format definition of new iCalendar elements is defined in [RFC5234].

   The ABNF syntax used by this document to specify the format
   definition of new message header fields to be used with the HTTP/1.1
   protocol is described in Section 2.1 of [RFC2616].  Since this
   Augmented BNF uses the basic production rules provided in Section 2.2
   of [RFC2616], these rules apply to this document as well.

   The term "protected" is used in the Conformance field of WebDAV
   property definitions as defined in Section 15 of [RFC4918].

   Calendaring and scheduling roles are referred to in quoted-strings of
   text with the first character of each word in uppercase.  For
   example, "Organizer" refers to a role of a calendar user within the
   scheduling protocol defined by [RFC5546].

1.3. XML Namespaces and Processing

   This document uses XML DTD fragments ([W3C.REC-xml-20081126],
   Section 3.2) as a purely notational convention.  WebDAV request and
   response bodies cannot be validated by a DTD due to the specific
   extensibility rules defined in Section 17 of [RFC4918] and due to the
   fact that all XML elements defined by that specification use the XML
   namespace name "DAV:".  In particular,

   1.  element names use the "DAV:" namespace,

2. element ordering is irrelevant unless explicitly stated,

   3.  extension elements (elements not already defined as valid child
       elements) can be added anywhere, except when explicitly stated
       otherwise, and

   4.  extension attributes (attributes not already defined as valid for
       this element) can be added anywhere, except when explicitly
       stated otherwise.

   The XML elements specified in this document are defined in the
   "urn:ietf:params:xml:ns:caldav" XML namespace registered by CalDAV
   [RFC4791].

   When XML element types in the namespaces "DAV:" and
   "urn:ietf:params:xml:ns:caldav" are referenced in this document
   outside of the context of an XML fragment, the strings "DAV:" and
   "CALDAV:" will be prefixed to the element types, respectively.

   This document inherits, and sometimes extends, DTD productions from
   Section 14 of [RFC4918].

   Also note that some CalDAV XML element names are identical to WebDAV
   XML element names, though their namespace differs.  Care needs to be
   taken not to confuse the two sets of names.

2.  Scheduling Support

   A server that supports the features described in this document is
   REQUIRED to support the CalDAV "calendar-access" [RFC4791] feature.
   Servers include "calendar-auto-schedule" as a field in the DAV
   response header from an OPTIONS request on any resource that supports
   any scheduling operations, properties, privileges, or methods.

   This specification introduces new collection resource types that are
   used to manage scheduling object resources, and scheduling privileges
   (as per Section 6), as well as provide scheduling functionality.  It
   is the server's responsibility to create these collection resources,
   and clients have no way to create or delete them.

2.1. Scheduling Outbox Collection

   A scheduling Outbox collection is used as the target for busy time
   information requests, and to manage privileges that apply to outgoing
   scheduling requests.

   A scheduling Outbox collection MUST report the DAV:collection and
   CALDAV:schedule-outbox XML elements in the value of the DAV:
   resourcetype property.  The element type declaration for CALDAV:
   schedule-outbox is

     <!ELEMENT schedule-outbox EMPTY>

   Example:

     <D:resourcetype xmlns:D="DAV:">
       <D:collection/>
       <C:schedule-outbox xmlns:C="urn:ietf:params:xml:ns:caldav"/>
     </D:resourcetype>

   A scheduling Outbox collection MUST NOT be a child (at any depth) of
   a calendar collection resource.

   The following WebDAV properties specified in CalDAV "calendar-access"
   [RFC4791] MAY also be defined on scheduling Outbox collections and
   apply to scheduling messages submitted to the scheduling Outbox
   collection with the POST method:

   o  CALDAV:supported-calendar-component-set

   o  CALDAV:supported-calendar-data

   o  CALDAV:max-resource-size

   o  CALDAV:min-date-time

   o  CALDAV:max-date-time

   o  CALDAV:max-attendees-per-instance

   The use of child resources in a scheduling Outbox collection is
   reserved for future revisions or extensions of this specification.

   The following WebDAV property is defined on principal resources and
   used to locate the corresponding Outbox collection for the associated
   principal.

2.1.1. CALDAV:schedule-outbox-URL Property

   Name:  schedule-outbox-URL

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Identify the URL of the scheduling Outbox collection owned
      by the associated principal resource.

   Protected:  This property MAY be protected.

   PROPFIND behavior:  This property SHOULD NOT be returned by a
      PROPFIND DAV:allprop request (as defined in Section 14.2 of
      [RFC4918]).

   COPY/MOVE behavior:  This property value SHOULD be preserved in COPY
      and MOVE operations.

   Description:  This property is needed for a client to determine where
      the scheduling Outbox collection of the current user is located so
      that sending of scheduling messages can occur.  If not present,
      then the associated calendar user is not enabled for the sending
      of scheduling messages on the server.

   Definition:

     <!ELEMENT schedule-outbox-URL (DAV:href)>

2.2. Scheduling Inbox Collection

   A scheduling Inbox collection contains copies of incoming scheduling
   messages.  These can be requests sent by an "Organizer", or replies
   sent by an "Attendee" in response to a request.  The scheduling Inbox
   collection is also used to manage scheduling privileges.

   A scheduling Inbox collection MUST report the DAV:collection and
   CALDAV:schedule-inbox XML elements in the value of the DAV:
   resourcetype property.  The element type declaration for CALDAV:
   schedule-inbox is

     <!ELEMENT schedule-inbox EMPTY>

   Example:

     <D:resourcetype xmlns:D="DAV:">
       <D:collection/>
       <C:schedule-inbox xmlns:C="urn:ietf:params:xml:ns:caldav"/>
     </D:resourcetype>

   Scheduling Inbox collections MUST only contain calendar object
   resources that obey the restrictions specified in iTIP [RFC5546].
   Consequently, scheduling Inbox collections MUST NOT contain any types
   of collection resources.  Restrictions defined in Section 4.1 of
   CalDAV "calendar-access" [RFC4791] on calendar object resources
   contained in calendar collections (e.g., Unique Identifier ("UID")
   uniqueness) do not apply to calendar object resources contained in a
   scheduling Inbox collection.  Thus, multiple calendar object
   resources contained in a scheduling Inbox collection can have the
   same "UID" property value (i.e., multiple scheduling messages for the
   same calendar component).

   A scheduling Inbox collection MUST NOT be a child (at any depth) of a
   calendar collection resource.

   The following WebDAV properties specified in CalDAV "calendar-access"
   [RFC4791] MAY also be defined on scheduling Inbox collections and
   apply to scheduling messages delivered to the collection:

   o  CALDAV:supported-calendar-component-set

   o  CALDAV:supported-calendar-data

   o  CALDAV:max-resource-size

   o  CALDAV:min-date-time

   o  CALDAV:max-date-time

   o  CALDAV:max-instances

   o  CALDAV:max-attendees-per-instance

   o  CALDAV:calendar-timezone

   The following WebDAV property is defined on principal resources and
   used to locate the corresponding Inbox collection for the associated
   principal.

2.2.1. CALDAV:schedule-inbox-URL Property

   Name:  schedule-inbox-URL

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Identify the URL of the scheduling Inbox collection owned
      by the associated principal resource.

   Protected:  This property MAY be protected.

   PROPFIND behavior:  This property SHOULD NOT be returned by a
      PROPFIND DAV:allprop request (as defined in Section 14.2 of
      [RFC4918]).

   COPY/MOVE behavior:  This property value SHOULD be preserved in COPY
      and MOVE operations.

   Description:  This property allows a client to determine where the
      scheduling Inbox collection of the current user is located so that
      processing of scheduling messages can occur.  If not present, then
      the associated calendar user is not enabled for reception of
      scheduling messages on the server.

   Definition:

     <!ELEMENT schedule-inbox-URL (DAV:href)>

2.3. Calendaring Reports Extensions

   This specification extends the CALDAV:calendar-query and CALDAV:
   calendar-multiget REPORTs to return results for calendar object
   resources in scheduling Inbox collections.

   When a CALDAV:calendar-query REPORT includes a time-range query and
   targets a scheduling Inbox collection, if any calendar object
   resources contain "VEVENT" calendar components that do not include a
   "DTSTART" iCalendar property (as allowed by iTIP [RFC5546]) then such
   components MUST always match the time-range query test.

   Note that the CALDAV:free-busy-query REPORT is not supported on
   scheduling Inbox collections.

2.4. Additional Principal Properties

   This section defines new properties for WebDAV principal resources as
   defined in [RFC3744].  These properties are likely to be protected,
   but the server MAY allow them to be written by appropriate users.

2.4.1. CALDAV:calendar-user-address-set Property

   Name:  calendar-user-address-set

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Identify the calendar addresses of the associated principal
      resource.

   Protected:  This property MAY be protected.

   PROPFIND behavior:  This property SHOULD NOT be returned by a
      PROPFIND DAV:allprop request (as defined in Section 14.2 of
      [RFC4918]).

   COPY/MOVE behavior:  This property value SHOULD be preserved in COPY
      and MOVE operations.

   Description:  Support for this property is REQUIRED.  This property
      is needed to map calendar user addresses in iCalendar data to
      principal resources and their associated scheduling Inbox and
      Outbox collections.  In the event that a user has no well-defined
      identifier for his calendar user address, the URI of his principal
      resource can be used.  This property SHOULD be searchable using
      the DAV:principal-property-search REPORT.  The DAV:principal-
      search-property-set REPORT SHOULD identify this property as such.
      If not present, then the associated calendar user is not enabled
      for scheduling on the server.

   Definition:

     <!ELEMENT calendar-user-address-set (DAV:href*)>

   Example:

     <C:calendar-user-address-set xmlns:D="DAV:"
         xmlns:C="urn:ietf:params:xml:ns:caldav">
       <D:href>mailto:bernard@example.com</D:href>
       <D:href>mailto:bernard.desruisseaux@example.com</D:href>
     </C:calendar-user-address-set>

2.4.2. CALDAV:calendar-user-type Property

   Name:  calendar-user-type

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Identifies the calendar user type of the associated
      principal resource.

   Value:  Same values allowed for the iCalendar "CUTYPE" property
      parameter defined in Section 3.2.3 of [RFC5545].

   Protected:  This property MAY be protected.

   PROPFIND behavior:  This property SHOULD NOT be returned by a
      PROPFIND DAV:allprop request (as defined in Section 14.2 of
      [RFC4918]).

   COPY/MOVE behavior:  This property value SHOULD be preserved in COPY
      and MOVE operations.

   Description:  Clients can query principal resources in order to look
      up "Attendees" available on the server.  When doing this, it is
      useful to know, or restrict the query to, certain types of
      calendar users (e.g., only search for "people", or only search for
      "rooms").  This property MAY be defined on principal resources to
      indicate the type of calendar user associated with the principal
      resource.  Its value is the same as the iCalendar "CUTYPE"
      property parameter that can be used on "ATTENDEE" properties.
      This property SHOULD be searchable using the DAV:principal-
      property-search REPORT.  The DAV:principal-search-property-set
      REPORT SHOULD identify this property as such.

   Definition:

     <!ELEMENT calendar-user-type (#PCDATA)>

   Example:

     <C:calendar-user-type
         xmlns:C="urn:ietf:params:xml:ns:caldav">INDIVIDUAL<
     /C:calendar-user-type>

3.  Scheduling Operations

   When a calendar object resource is created, modified, or removed from
   a calendar collection, the server examines the calendar data and
   checks to see whether the data represents a scheduling object
   resource.  If it does, the server will automatically attempt to
   deliver a scheduling message to the appropriate calendar users.
   Several types of scheduling operations can occur in this case,
   equivalent to iTIP "REQUEST", "REPLY", "CANCEL", and "ADD"
   operations.

3.1. Identifying Scheduling Object Resources

   Calendar object resources on which the server performs scheduling
   operations are referred to as scheduling object resources.  There are
   two types of scheduling object resources: organizer scheduling object
   resources, and attendee scheduling object resources.

   A calendar object resource is considered to be a valid organizer
   scheduling object resource if the "ORGANIZER" iCalendar property is
   present and set in all the calendar components to a value that
   matches one of the calendar user addresses of the owner of the
   calendar collection.

   A calendar object resource is considered to be a valid attendee
   scheduling object resource if the "ORGANIZER" iCalendar property is
   present and set in all the calendar components to the same value and
   doesn't match one of the calendar user addresses of the owner of the
   calendar collection, and if at least one of the "ATTENDEE" iCalendar
   property values matches one of the calendar user addresses of the
   owner of the calendar collection.

   The creation of attendee scheduling object resources is typically
   done by the server, with the resource being created in an appropriate
   calendar collection (see Section 4.3).

3.2. Handling Scheduling Object Resources

   The server's behavior when processing a scheduling object resource
   depends on whether it is owned by the "Organizer" or an "Attendee"
   specified in the calendar data.

3.2.1. Organizer Scheduling Object Resources

   An "Organizer" can create, modify, or remove a scheduling object
   resource, subject to access privileges, preconditions, and the
   restrictions defined in Section 4.1 of [RFC4791].  These operations
   are each described next, and how they are invoked via HTTP requests
   is described in Section 3.2.3.

   The "Organizer" of a calendar component can also be an "Attendee" of
   that calendar component.  In such cases, the server MUST NOT send a
   scheduling message to the "Attendee" that matches the "Organizer".

   The server SHOULD reject any attempt to set the "PARTSTAT" iCalendar
   property parameter value of the "ATTENDEE" iCalendar property of
   other users in the calendar object resource to a value other than
   "NEEDS-ACTION" if the "SCHEDULE-AGENT" property parameter value is
   not present or set to the value "SERVER".

   The server MAY reject attempts to create a scheduling object resource
   that specifies a "UID" property value already specified in a
   scheduling object resource contained in another calendar collection
   of the "Organizer".

3.2.1.1. Create

   When an "Organizer" creates a scheduling object resource, the server
   MUST inspect each "ATTENDEE" property to determine whether to send a
   scheduling message.  The table below indicates the appropriate iTIP
   method used by the server, taking into account any "SCHEDULE-AGENT"
   property parameter (see Section 7.1) specified on each "ATTENDEE"
   property.

                    +------------------+-------------+
                    | SCHEDULE-AGENT   | iTIP METHOD |
                    +------------------+-------------+
                    | SERVER (default) | REQUEST     |
                    |                  |             |
                    | CLIENT           | --          |
                    |                  |             |
                    | NONE             | --          |
                    +------------------+-------------+

   "SCHEDULE-STATUS" iCalendar property parameters are added or changed
   on "ATTENDEE" iCalendar properties in the scheduling object resource
   being created as described in Section 7.3, with the value set as
   described in Section 3.2.9.  This will result in the created calendar
   object resource differing from the calendar data sent in the HTTP
   request.  As a result, clients MAY reload the calendar data from the
   server in order to update to the new server-generated state
   information.

   The server MUST add a "SCHEDULE-STATUS" iCalendar property parameter
   (see Section 7.3) to the "ATTENDEE" iCalendar property in the
   scheduling object resource being created, and set its value as
   described in Section 3.2.9.  This will result in the created calendar
   object resource differing from the calendar data sent in the HTTP
   request.  As a result, clients MAY reload the calendar data from the
   server in order to update to the new server-generated state
   information.  Servers MUST NOT set the "SCHEDULE-STATUS" property
   parameter on the "ATTENDEE" property of "Attendees" for which it did
   not attempt to deliver a scheduling message.

   The server MUST return an error with the CALDAV:allowed-organizer-
   scheduling-object-change precondition code (Section 3.2.4.3) when the
   "Organizer" attempts to change the iCalendar data in a manner that is
   forbidden.

3.2.1.2. Modify

   When an "Organizer" modifies a scheduling object resource, the server
   MUST inspect each "ATTENDEE" property in both the original and
   modified iCalendar data on a per-instance basis to determine whether
   to send a scheduling message.  The table below indicates the
   appropriate iTIP method used by the server, taking into account any
   "SCHEDULE-AGENT" property parameter (see Section 7.1) specified on
   each "ATTENDEE" property.  The values "SERVER", "CLIENT", and "NONE"
   in the top and left titles of the table refer to the "SCHEDULE-AGENT"
   parameter value of the "ATTENDEE" property, and the values "<Absent>"
   and "<Removed>" are used to cover the cases where the "ATTENDEE"
   property is not present (Original) or is being removed (Modified).

   +---------------+-----------------------------------------------+
   |               |                   Modified                    |
   |               +-----------+-----------+-----------+-----------+
   |               | <Removed> | SERVER    | CLIENT    | NONE      |
   |               |           | (default) |           |           |
   +===+===========+===========+===========+===========+===========+
   |   | <Absent>  |  --       | REQUEST / | --        | --        |
   | O |           |           | ADD       |           |           |
   | r +-----------+-----------+-----------+-----------+-----------+
   | i | SERVER    |  CANCEL   | REQUEST   | CANCEL    | CANCEL    |
   | g | (default) |           |           |           |           |
   | i +-----------+-----------+-----------+-----------+-----------+
   | n | CLIENT    |  --       | REQUEST / | --        | --        |
   | a |           |           | ADD       |           |           |
   | l +-----------+-----------+-----------+-----------+-----------+
   |   | NONE      |  --       | REQUEST / | --        | --        |
   |   |           |           | ADD       |           |           |
   +---+-----------+-----------+-----------+-----------+-----------+

   "SCHEDULE-STATUS" iCalendar property parameters are added or changed
   on "ATTENDEE" iCalendar properties in the scheduling object resource
   being modified as described in Section 7.3, with the value set as
   described in Section 3.2.9.  This will result in the created calendar
   object resource differing from the calendar data sent in the HTTP
   request.  As a result, clients MAY reload the calendar data from the
   server in order to update to the new server-generated state
   information.

   The server MUST return an error with the CALDAV:allowed-organizer-
   scheduling-object-change precondition code (Section 3.2.4.3) when the
   "Organizer" attempts to change the iCalendar data in a manner that is
   forbidden.

3.2.1.3. Remove

   When an "Organizer" removes a scheduling object resource, the server
   MUST inspect each "ATTENDEE" property to determine whether to send a
   scheduling message.  The table below indicates the appropriate iTIP
   method used by the server, taking into account any "SCHEDULE-AGENT"
   property parameter (see Section 7.1) specified on each "ATTENDEE"
   property.

                    +------------------+-------------+
                    | SCHEDULE-AGENT   | iTIP METHOD |
                    +------------------+-------------+
                    | SERVER (default) | CANCEL      |
                    |                  |             |
                    | CLIENT           | --          |
                    |                  |             |
                    | NONE             | --          |
                    +------------------+-------------+

3.2.2. Attendee Scheduling Object Resources

   An "Attendee" can create, modify, or remove a scheduling object
   resource.  These operations are each described next, and how they are
   invoked via HTTP requests is described in Section 3.2.3.

3.2.2.1. Allowed "Attendee" Changes

   "Attendees" are allowed to make some changes to a scheduling object
   resource, though key properties such as start time, end time,
   location, and summary are typically under the control of the
   "Organizer".

   Servers MUST allow "Attendees" to make the following iCalendar data
   changes, subject to other restrictions, such as access privileges and
   preconditions:

   1.   change their own "PARTSTAT" iCalendar property parameter value.

   2.   add, modify, or remove any "TRANSP" iCalendar properties.

   3.   add, modify, or remove any "PERCENT-COMPLETE" iCalendar
        properties.

   4.   add, modify, or remove any "COMPLETED" iCalendar properties.

5. add, modify, or remove any "VALARM" iCalendar components.

6. add, modify, or remove the "CALSCALE" iCalendar property within
the top-level "VCALENDAR" component.

7. modify the "PRODID" iCalendar property within the top-level
"VCALENDAR" component.

8. add "EXDATE" iCalendar properties and possibly remove components
for overridden recurrence instances.

9. add, modify, or remove any "CREATED", "DTSTAMP", and
"LAST-MODIFIED" iCalendar properties.

   10.  add, modify, or remove "SCHEDULE-STATUS" iCalendar property
        parameters on "ATTENDEE" properties that have a "SCHEDULE-AGENT"
        parameter set to "CLIENT".

   11.  add new components to represent overridden recurrence instances,
        provided the only changes to the recurrence instance follow the
        rules above.

   The server MUST return an error with the CALDAV:allowed-attendee-
   scheduling-object-change precondition code (Section 3.2.4.4) when the
   "Attendee" attempts to change the iCalendar data in a manner
   forbidden by the server.

3.2.2.2. Create

   Typically, an "Attendee" does not create scheduling object resources,
   as scheduling messages delivered to him on the server are
   automatically processed by the server and placed on one of his
   calendars (see Section 4).  However, in some cases, a scheduling
   message can get delivered directly to the client (e.g., via email
   [RFC6047]), and the "Attendee" might wish to store that on the
   server.  In that case, the client creates a scheduling object
   resource in a calendar belonging to the "Attendee".  It can then set
   the "SCHEDULE-AGENT" iCalendar property parameter on all "ORGANIZER"
   iCalendar properties in the resource to determine how the server
   treats the resource.  The value of the "SCHEDULE-AGENT" iCalendar
   property parameter on all "ORGANIZER" iCalendar properties MUST be
   the same.

   +----------------+--------------------------------------------------+
   | SCHEDULE-AGENT | Action                                           |
   +----------------+--------------------------------------------------+
   | SERVER         | The server will attempt to process changes to    |
   | (default)      | the resource using the normal rules for attendee |
   |                | scheduling object resources.                     |
   |                |                                                  |
   | CLIENT         | The server does no special processing of the     |
   |                | resource.  The client is assumed to be handling  |
   |                | "Attendee" replies, etc.                         |
   |                |                                                  |
   | NONE           | The server does no special processing of the     |
   |                | resource.                                        |
   +----------------+--------------------------------------------------+

   "SCHEDULE-STATUS" iCalendar property parameters are added or changed
   on "ORGANIZER" iCalendar properties in the scheduling object resource
   being created as described in Section 7.3, with the value set as
   described in Section 3.2.9.

3.2.2.3. Modify

   When a scheduling object resource is modified by an "Attendee", the
   server's behavior depends on the value of the "SCHEDULE-AGENT"
   iCalendar property parameter on the "ORGANIZER" iCalendar properties:

   +----------------+--------------------------------------------------+
   | SCHEDULE-AGENT | Action                                           |
   +----------------+--------------------------------------------------+
   | SERVER         | The server will attempt to process the update    |
   | (default)      | using the behavior listed below.                 |
   |                |                                                  |
   | CLIENT         | The server does no special processing of the     |
   |                | resource.  The client is assumed to be handling  |
   |                | any "Attendee" replies, etc.                     |
   |                |                                                  |
   | NONE           | The server does no special processing of the     |
   |                | resource.                                        |
   +----------------+--------------------------------------------------+

   The server will inspect the changes by comparing the new scheduling
   object resource with the existing scheduling object resource.

   If the "Attendee" changes one or more "PARTSTAT" iCalendar property
   values on any component, or adds an overridden component with a
   changed "PARTSTAT" property, then the server MUST deliver an iTIP
   "REPLY" scheduling message to the "Organizer" to indicate the new
   participation status of the "Attendee".

   If the "Attendee" adds an "EXDATE" property value to effectively
   remove a recurrence instance, the server MUST deliver an iTIP "REPLY"
   scheduling message to the "Organizer" to indicate that the "Attendee"
   has declined the instance.

   "SCHEDULE-STATUS" iCalendar property parameters are added or changed
   on "ORGANIZER" iCalendar properties in the scheduling object resource
   being modified as described in Section 7.3, with the value set as
   described in Section 3.2.9.  This will result in the updated calendar
   object resource differing from the calendar data sent in the HTTP
   request.  As a result, clients MAY reload the calendar data from the
   server in order to update to the new server-generated state
   information.

3.2.2.4. Remove

   When a scheduling object resource is removed by an "Attendee", the
   server's behavior depends on the value of the "SCHEDULE-AGENT"
   iCalendar property parameter on the "ORGANIZER" iCalendar properties:

   +----------------+--------------------------------------------------+
   | SCHEDULE-AGENT | Action                                           |
   +----------------+--------------------------------------------------+
   | SERVER         | The server will attempt to process the removal,  |
   | (default)      | taking into account any "Schedule-Reply" request |
   |                | header as per Section 8.1.                       |
   |                |                                                  |
   | CLIENT         | The server does no special processing of the     |
   |                | resource.  The client is assumed to be handling  |
   |                | any "Attendee" replies, etc.                     |
   |                |                                                  |
   | NONE           | The server does no special processing of the     |
   |                | resource.                                        |
   +----------------+--------------------------------------------------+

3.2.3. HTTP Methods

   This section describes how the use of various HTTP [RFC2616] and
   WebDAV [RFC4918] methods on a scheduling object resource will cause a
   create, modify, or remove operation on that resource as described
   above.  The use of these methods is subject to the restrictions in
   [RFC4791], in addition to what is described below.

3.2.3.1. PUT

3.2.3.2. DELETE

   When the server receives a DELETE method request targeted at a
   scheduling object resource, it MUST execute the Remove operation.

   When the server receives a DELETE method request targeted at a
   calendar collection, it MUST execute the Remove operation on all
   scheduling object resources contained in the calendar collection.

3.2.3.3. COPY

3.2.3.4. MOVE

   When the server receives a MOVE method request, it MUST execute the
   following operations based on the source and destination collections
   in the request:

   +-----------------------+------------------------+------------------+
   | Source Collection     | Destination Collection | Server Operation |
   +-----------------------+------------------------+------------------+
   | Non-calendar          | Non-calendar           | None             |
   | collection            | collection             |                  |
   |                       |                        |                  |
   | Non-calendar          | Calendar collection    | (1)              |
   | collection            |                        |                  |
   |                       |                        |                  |
   | Calendar collection   | Non-calendar           | (2)              |
   |                       | collection             |                  |
   |                       |                        |                  |
   | Calendar collection   | Calendar collection    | None             |
   +-----------------------+------------------------+------------------+

   Note (1):  The rules in Section 3.2.3.1 are applied for the
              destination of the MOVE request.

   Note (2):  The rules in Section 3.2.3.2 are applied for the source of
              the MOVE request.

   The behavior of a MOVE method request on a calendar collection is
   undefined.

3.2.4. Additional Method Preconditions

   This specification defines method preconditions (see Section 16 of
   WebDAV [RFC4918]), in addition to those in [RFC4791], to provide
   machine-parseable information in error responses.

3.2.4.1. CALDAV:unique-scheduling-object-resource Precondition

   Name:  unique-scheduling-object-resource

   Namespace:  urn:ietf:params:xml:ns:caldav

   Apply to:  PUT, COPY, and MOVE

   Use with:  403 Forbidden

   Purpose:  (precondition) -- Servers MAY reject requests to create a
      scheduling object resource with an iCalendar "UID" property value
      already in use by another scheduling object resource owned by the
      same user in other calendar collections.  Servers SHOULD report
      the URL of the scheduling object resource that is already making
      use of the same "UID" property value in the DAV:href element.

   Definition:

     <!ELEMENT unique-scheduling-object-resource (DAV:href?)>

   Example:

     <C:unique-scheduling-object-resource xmlns:D="DAV:"
         xmlns:C="urn:ietf:params:xml:ns:caldav">
       <D:href>/home/bernard/calendars/personal/abc123.ics</D:href>
     </C:unique-scheduling-object-resource>

3.2.4.2. CALDAV:same-organizer-in-all-components Precondition

   Name:  same-organizer-in-all-components

   Namespace:  urn:ietf:params:xml:ns:caldav

   Apply to:  PUT, COPY, and MOVE

   Use with:  403 Forbidden

   Purpose:  (precondition) -- All the calendar components in a
      scheduling object resource MUST contain the same "ORGANIZER"
      property value when present.

   Definition:

     <!ELEMENT same-organizer-in-all-components EMPTY>

3.2.4.3. CALDAV:allowed-organizer-scheduling-object-change Precondition

   Name:  allowed-organizer-scheduling-object-change

   Namespace:  urn:ietf:params:xml:ns:caldav

   Apply to:  PUT, COPY, and MOVE

   Use with:  403 Forbidden

   Purpose:  (precondition) -- Servers MAY impose restrictions on
      modifications allowed by an "Organizer".  For instance, servers
      MAY prevent the "Organizer" from setting the "PARTSTAT" property
      parameter to a value other than "NEEDS-ACTION" if the
      corresponding "ATTENDEE" property has the "SCHEDULE-AGENT"
      property parameter set to "SERVER", or does not have the
      "SCHEDULE-AGENT" property parameter.  See Section 3.2.1.

   Definition:

     <!ELEMENT allowed-organizer-scheduling-object-change EMPTY>

3.2.4.4. CALDAV:allowed-attendee-scheduling-object-change Precondition

   Name:  allowed-attendee-scheduling-object-change

   Namespace:  urn:ietf:params:xml:ns:caldav

   Apply to:  PUT, COPY, and MOVE

   Use with:  403 Forbidden

   Purpose:  (precondition) -- Servers MAY impose restrictions on
      modifications allowed by an "Attendee", subject to the allowed
      changes specified in Section 3.2.2.1.

   Definition:

     <!ELEMENT allowed-attendee-scheduling-object-change EMPTY>

3.2.5. DTSTAMP and SEQUENCE Properties

   The server MUST ensure that a "DTSTAMP" iCalendar property is present
   and set the value to the UTC time that the scheduling message was
   generated (as required by iCalendar).

   The server MUST ensure that for each type of scheduling operation,
   the "SEQUENCE" iCalendar property value is updated as per iTIP
   [RFC5546].

3.2.6. Restrict Recurrence Instances Sent to "Attendees"

   Servers MUST ensure that "Attendees" only get information about
   recurrence instances that explicitly include them as an "Attendee",
   when delivering scheduling messages for recurring calendar
   components.

   For example, if an "Attendee" is invited to only a single instance of
   a recurring event, the organizer scheduling object resource will
   contain an overridden instance in the form of a separate calendar
   component.  That separate calendar component will include the
   "ATTENDEE" property referencing the "one-off" "Attendee".  That
   "Attendee" will not be listed in any other calendar components in the
   scheduling object resource.  Any scheduling messages delivered to the
   "Attendee" will only contain information about this overridden
   instance.

   As another example, an "Attendee" could be excluded from one instance
   of a recurring event.  In that case, the organizer scheduling object
   resource will include an overridden instance with an "ATTENDEE" list
   that does not include the "Attendee" being excluded.  Any scheduling
   messages delivered to the "Attendee" will not specify the overridden
   instance but rather will include an "EXDATE" property in the "master"
   component that defines the recurrence set.

3.2.7. Forcing the Server to Send a Scheduling Message

   The iCalendar property parameter "SCHEDULE-FORCE-SEND", defined in
   Section 7.2, can be used by a calendar user to force the server to
   send a scheduling message to an "Attendee" or the "Organizer" in a
   situation where the server would not normally send a scheduling
   message.  For instance, an "Organizer" could use this property
   parameter to request an "Attendee" that previously declined an
   invitation to reconsider his participation status without being
   forced to modify the event.

3.2.8. "Attendee" Participation Status

   This section specifies additional requirements on the handling of the
   "PARTSTAT" property parameter when the "SCHEDULE-AGENT" property
   parameter on the corresponding "ATTENDEE" property is set to the
   value "SERVER" or is not present.

   A reschedule occurs when any "DTSTART", "DTEND", "DURATION", "DUE",
   "RRULE", "RDATE", or "EXDATE" property changes in a calendar
   component such that existing recurrence instances are impacted by the
   changes, as shown in the table below.  Servers MUST reset the
   "PARTSTAT" property parameter value of all "ATTENDEE" properties,
   except the one that corresponds to the "Organizer", to "NEEDS-ACTION"
   for each calendar component change that causes any instance to be
   rescheduled.

   +-----------+-------------------------------------------------------+
   | Property  | Server Action                                         |
   +-----------+-------------------------------------------------------+
   | DTSTART,  | Any change to these properties results in "PARTSTAT"  |
   | DTEND,    | being set to "NEEDS-ACTION".                          |
   | DURATION, |                                                       |
   | DUE       |                                                       |
   |           |                                                       |
   | RRULE     | A change to or addition of this property that results |
   |           | in the addition of new recurring instances or a       |
   |           | change in time for existing recurring instances       |
   |           | results in "PARTSTAT" being reset to "NEEDS-ACTION"   |
   |           | on each affected component.                           |
   |           |                                                       |
   | RDATE     | A change to or addition of this property that results |
   |           | in the addition of new recurring instances or a       |
   |           | change in time for existing recurring instances       |
   |           | results in "PARTSTAT" being reset to "NEEDS-ACTION"   |
   |           | on each affected component.                           |
   |           |                                                       |
   | EXDATE    | A change to or removal of this property that results  |
   |           | in the reinstatement of recurring instances results   |
   |           | in "PARTSTAT" being set to "NEEDS-ACTION" on each     |
   |           | affected component.                                   |
   +-----------+-------------------------------------------------------+

   The server MAY allow the "Organizer's" client to change an
   "Attendee's" "PARTSTAT" property parameter value to "NEEDS-ACTION" at
   any other time (e.g., when the "LOCATION" property value changes, an
   "Organizer" might wish to re-invite "Attendees" who might be impacted
   by the change).

3.2.9. Schedule Status Values

   When scheduling with an "Attendee", there are two types of status
   information that can be returned during the operation.  The first
   type of status information is a "delivery" status that indicates
   whether the scheduling message from the "Organizer" to the "Attendee"
   was delivered or not, or what the current status of delivery is.  The
   second type of status information is a "reply" status corresponding
   to the "Attendee's" own "REQUEST-STATUS" information from the
   scheduling message reply that is sent back to the "Organizer".

   Similarly, when an "Attendee" sends a reply back to the "Organizer",
   there will be "delivery" status information for the scheduling
   message sent to the "Organizer".  However, there is no
   "REQUEST-STATUS" sent back by the "Organizer", so there is no
   equivalent of the "reply" status as per scheduling messages to
   "Attendees".

   The "delivery" status information on an "ORGANIZER" or "ATTENDEE"
   iCalendar property is conveyed in the "SCHEDULE-STATUS" property
   parameter value (Section 7.3).  The status code value for "delivery"
   status can be one of the following:

   +----------+--------------------------------------------------------+
   | Delivery | Description                                            |
   | Status   |                                                        |
   | Code     |                                                        |
   +----------+--------------------------------------------------------+
   | 1.0      | The scheduling message is pending.  That is, the       |
   |          | server is still in the process of sending the message. |
   |          | The status code value can be expected to change once   |
   |          | the server has completed its sending and delivery      |
   |          | attempts.                                              |
   |          |                                                        |
   | 1.1      | The scheduling message has been successfully sent.     |
   |          | However, the server does not have explicit information |
   |          | about whether the scheduling message was successfully  |
   |          | delivered to the recipient.  This state can occur with |
   |          | "store and forward" style scheduling protocols such as |
   |          | iMIP [RFC6047] (iTIP using email).                     |
   |          |                                                        |
   | 1.2      | The scheduling message has been successfully           |
   |          | delivered.                                             |
   |          |                                                        |

   | 3.7      | The scheduling message was not delivered because the   |
   |          | server did not recognize the calendar user address as  |
   |          | a valid calendar user.  Note that this code applies to |
   |          | both "Organizer" and "Attendee" calendar user          |
   |          | addresses.                                             |
   |          |                                                        |
   | 3.8      | The scheduling message was not delivered due to        |
   |          | insufficient privileges.  Note that this code applies  |
   |          | to privileges granted by both the "Organizer" and      |
   |          | "Attendee" calendar users.                             |
   |          |                                                        |
   | 5.1      | The scheduling message was not delivered because the   |
   |          | server could not complete delivery of the message.     |
   |          | This is likely due to a temporary failure, and the     |
   |          | originator can try to send the message again at a      |
   |          | later time.                                            |
   |          |                                                        |
   | 5.2      | The scheduling message was not delivered because the   |
   |          | server was not able to find a way to deliver the       |
   |          | message.  This is likely a permanent failure, and the  |
   |          | originator ought not try to send the message again, at |
   |          | least without verifying/correcting the calendar user   |
   |          | address of the recipient.                              |
   |          |                                                        |
   | 5.3      | The scheduling message was not delivered and was       |
   |          | rejected because scheduling with that recipient is not |
   |          | allowed.  This is likely a permanent failure, and the  |
   |          | originator ought not try to send the message again.    |
   +----------+--------------------------------------------------------+

   The status code for "reply" status can be any of the valid iTIP
   [RFC5546] "REQUEST-STATUS" values.

   The 1.xx "REQUEST-STATUS" codes are new.  This specification modifies
   item (2) of Section 3.6 of [RFC5546] by adding the following
   restriction:

      For a 1.xx code, all components MUST have exactly the same code.

Definition of the new 1.xx codes is as follows:

3.2.9.1. Status Code 1.0

   Status Code:  1.0

   Status Description:  Pending.

   Status Exception Data:  None.

   Description:  Delivery of the iTIP message is pending.

3.2.9.2. Status Code 1.1

   Status Code:  1.1

   Status Description:  Sent.

   Status Exception Data:  None.

   Description:  The iTIP message has been sent, though no information
      about successful delivery is known.

3.2.9.3. Status Code 1.2

   Status Code:  1.2

   Status Description:  Delivered.

   Status Exception Data:  None.

   Description:  The iTIP message has been sent and delivered.

3.2.10. Avoiding Conflicts when Updating Scheduling Object Resources

   Scheduling object resources on the server might change frequently as
   "Attendees" change their participation status, triggering updates to
   the "Organizer", and refreshes of other "Attendees'" copies of the
   scheduling object resource.  This can lead to an "inconsequential"
   change to a calendar user's data -- one that does not directly impact
   the user's own participation status.  When this occurs, clients have
   to reload calendar data and reconcile with changes being made by
   calendar users.  To avoid the need for this, the server can instead
   merge calendar data changes from a client with changes made as a
   result of a scheduling operation carried out by some other calendar
   user.

   This specification introduces a new WebDAV resource property CALDAV:
   schedule-tag with a corresponding response header "Schedule-Tag", and
   a new "If-Schedule-Tag-Match" request header to allow client changes
   to be appropriately merged with server changes in the case where the
   changes on the server were the result of an "inconsequential"
   scheduling message update (one that simply updates the status
   information of "Attendees" due to a reply from another "Attendee").

   Servers MUST automatically resolve conflicts with "inconsequential"
   changes done to scheduling object resources when the "If-Schedule-
   Tag-Match" request header is specified.  The If-Schedule-Tag-Match
   request header applies only to the Request-URI, and not to the
   destination of a COPY or MOVE.

   A response to any successful GET or PUT request targeting a
   scheduling object resource MUST include a Schedule-Tag response
   header with the value set to the same value as the CALDAV:schedule-
   tag WebDAV property of the resource.

   A response to any successful COPY or MOVE request that specifies a
   Destination request header targeting a scheduling object resource
   MUST include a Schedule-Tag response header with the value set to the
   same value as the CALDAV:schedule-tag WebDAV property of the
   destination resource.

   Clients SHOULD use the If-Schedule-Tag-Match header on requests that
   update scheduling object resources, instead of HTTP ETag-based
   precondition tests (e.g., If-Match).  Normal ETag-based precondition
   tests are used in all other cases, e.g., for synchronization.

   The value of the CALDAV:schedule-tag property changes according to
   these rules:

   o  For an "Organizer's" copy of a scheduling object resource:

      1.  The server MUST NOT change the CALDAV:schedule-tag property
          value when the scheduling object resource is updated as the
          result of automatically processing a scheduling message reply
          from an "Attendee".  For instance, when an "Attendee" replies
          to the "Organizer", the CALDAV:schedule-tag property is
          unchanged after the "Organizer's" scheduling object resource
          has been automatically updated by the server with the
          "Attendee's" new participation status.

      2.  The server MUST change the CALDAV:schedule-tag property value
          when the scheduling object resource is changed directly via an
          HTTP request (e.g., PUT, COPY, or MOVE).

   o  For an "Attendee's" copy of a scheduling object resource:

      1.  The server MUST change the CALDAV:schedule-tag property value
          when the scheduling object resource is changed as the result
          of processing a scheduling message update from an "Organizer"
          that contains changes other than just the participation status
          of "Attendees".

      2.  The server MUST NOT change the CALDAV:schedule-tag property
          value when the scheduling object resource is changed as the
          result of processing a scheduling message update from an
          "Organizer" that only specifies changes in the participation
          status of "Attendees".  For instance, when "Attendee" "A"
          replies to "Organizer" "O", and "Attendee" "B" receives a
          scheduling message update from "Organizer" "O" with the new
          participation status of "Attendee" "A", the CALDAV:schedule-
          tag property of "Attendee" "B"'s scheduling object resource
          would remain the same.

      3.  The server MUST change the CALDAV:schedule-tag property value
          when the scheduling object resource is changed directly via an
          HTTP request (e.g., PUT, COPY, or MOVE).

3.2.10.1. PUT

   Clients MAY use the If-Schedule-Tag-Match request header to do a PUT
   request that ensures that "inconsequential" changes on the server do
   not result in a precondition error.  The value of the request header
   is set to the last Schedule-Tag value received for the resource being
   modified.  If the value of the If-Schedule-Tag-Match header matches
   the current value of the CALDAV:schedule-tag property, the server
   MUST take any "ATTENDEE" property changes for all "Attendees" other
   than the owner of the scheduling object resource and apply those to
   the new resource being stored.  Otherwise, the server MUST fail the
   request with a 412 Precondition Failed status code.

3.2.10.2. DELETE, COPY, or MOVE

   Clients MAY use the If-Schedule-Tag-Match request header to do a
   DELETE, COPY, or MOVE request that ensures that "inconsequential"
   changes on the server do not result in a precondition error.  The
   value of the request header is set to the last Schedule-Tag value
   received for the resource being deleted.  If the value of the
   If-Schedule-Tag-Match header matches the current value of the CALDAV:
   schedule-tag property, the server performs the normal DELETE, COPY,
   or MOVE request processing for the resource.  Otherwise, the server
   MUST fail the request with a 412 Precondition Failed status code.

4.  Processing Incoming Scheduling Messages

   Scheduling operations can cause the delivery of a scheduling message
   into an "Organizer's" or "Attendee's" scheduling Inbox collection.
   Servers MUST automatically process incoming scheduling messages using
   the rules defined by [RFC5546], by creating or updating the
   corresponding scheduling object resources on calendars owned by the
   owner of the scheduling Inbox collection.  In addition, the
   scheduling message is stored in the scheduling Inbox collection as an
   indicator to the client that a scheduling operation has taken place.
   Scheduling messages are typically removed from the scheduling Inbox
   collection by the client once the calendar user has acknowledged the
   change.

   The server MUST take into account privileges on the scheduling Inbox
   collection when processing incoming scheduling messages, to determine
   whether delivery of the scheduling message is allowed.  Privileges on
   calendars containing any matching scheduling object resource are not
   considered in this case (i.e., a schedule message from another user
   can cause modifications to resources in calendar collections that the
   other user would not normally have read or write access to).
   Additionally, servers MUST take into account any scheduling Inbox
   collection preconditions (see Section 2.2) when delivering the
   scheduling message, and MUST take into account the similar
   preconditions on any calendar collection that contains, or would
   contain, the corresponding scheduling object resource.

4.1. Processing "Organizer" Requests, Additions, and Cancellations

   For a scheduling message sent by an "Organizer", the server first
   tries to locate a corresponding scheduling object resource belonging
   to the "Attendee".  If no matching scheduling object resource exists,
   the server treats the scheduling message as a new message; otherwise,
   it is treated as an update.

   In the case of a new message, the server processes the scheduling
   message and creates a new scheduling object resource as per
   Section 4.3.

   In the case of an update, the server processes the scheduling message
   and updates the matching scheduling object resource belonging to the
   "Attendee" to reflect the changes sent by the "Organizer".

   In each case, the scheduling message MUST only appear in the
   "Attendee's" scheduling Inbox collection once all automatic
   processing has been done.

4.2. Processing "Attendee" Replies

   For a scheduling message reply sent by an "Attendee", the server
   first locates the corresponding scheduling object resource belonging
   to the "Organizer".  If the corresponding scheduling object resource
   cannot be found, the server SHOULD ignore the scheduling message.

   The server MUST then update the "PARTSTAT" iCalendar property
   parameter value of each "ATTENDEE" iCalendar property in the
   scheduling object resource to match the changes indicated in the
   reply (taking into account the fact that an "Attendee" could have
   created a new overridden iCalendar component to indicate different
   participation status on one or more instances of a recurring event).

   The server MUST also update or add the "SCHEDULE-STATUS" property
   parameter on each matching "ATTENDEE" iCalendar property and set its
   value to that of the "REQUEST-STATUS" property in the reply, or to
   "2.0" if "REQUEST-STATUS" is not present (also taking into account
   recurrence instances).  If there are multiple "REQUEST-STATUS"
   properties in the reply, the "SCHEDULE-STATUS" property parameter
   value is set to a comma-separated list of status codes, one from each
   "REQUEST-STATUS" property.

   The server SHOULD send scheduling messages to all the other
   "Attendees" indicating the change in participation status of the
   "Attendee" replying, subject to the recurrence requirements of
   Section 3.2.6.

   The scheduling message MUST only appear in the "Organizer's"
   scheduling Inbox collection once all automatic processing has been
   done.

4.3. Default Calendar Collection

   The server processes scheduling messages received for an "Attendee"
   by creating a new scheduling object resource in a calendar collection
   belonging to the "Attendee", when one does not already exist.  A
   calendar user that is an "Attendee" in a scheduling operation MUST
   have at least one valid calendar collection available.  If there is
   no valid calendar collection, then the server MUST reject the attempt
   to deliver the scheduling message to the "Attendee".

   Servers MAY provide support for a default calendar collection -- that
   is, the calendar collection in which new scheduling object resources
   will be created.  The CALDAV:schedule-default-calendar-URL WebDAV
   property, which can be present on the scheduling Inbox collection of
   a calendar user, specifies whether this calendar user has a default
   calendar collection.  See Section 9.2.

   Servers SHOULD create new scheduling object resources in the default
   calendar collection, if the CALDAV:schedule-default-calendar-URL
   WebDAV property is set.

   Servers MAY allow clients to change the default calendar collection
   by changing the value of the CALDAV:schedule-default-calendar-URL
   WebDAV property on the scheduling Inbox collection.  However, the
   server MUST ensure that any new value for that property refers to a
   valid calendar collection belonging to the owner of the scheduling
   Inbox collection.

   Servers MUST reject any attempt to delete the default calendar
   collection.

4.3.1. Additional Method Preconditions

   This specification defines additional method preconditions (see
   Section 16 of WebDAV [RFC4918]) to provide machine-parseable
   information in error responses.

4.3.1.1. CALDAV:default-calendar-needed Precondition

   Name:  default-calendar-needed

   Namespace:  urn:ietf:params:xml:ns:caldav

   Apply to:  DELETE

   Use with:  403 Forbidden

   Purpose:  (precondition) -- The client attempted to delete the
      calendar collection currently referenced by the CALDAV:schedule-
      default-calendar-URL property, or attempted to remove the CALDAV:
      schedule-default-calendar-URL property on the scheduling Inbox
      collection on a server that doesn't allow such operations.

   Definition:

     <!ELEMENT default-calendar-needed EMPTY>

4.3.1.2. CALDAV:valid-schedule-default-calendar-URL Precondition

   Name:  valid-schedule-default-calendar-URL

   Namespace:  urn:ietf:params:xml:ns:caldav

   Apply to:  PROPPATCH

   Use with:  403 Forbidden

   Purpose:  (precondition) -- The client attempted to set the CALDAV:
      schedule-default-calendar-URL property to a DAV:href element that
      doesn't reference a valid calendar collection.  Note: Servers that
      do not allow clients to change the CALDAV:schedule-default-
      calendar-URL property would simply return the DAV:cannot-modify-
      protected-property precondition defined in Section 16 of WebDAV
      [RFC4918].

   Definition:

     <!ELEMENT valid-schedule-default-calendar-URL EMPTY>

5.  Request for Busy Time Information

   Busy time information of one or more calendar users can be determined
   by submitting a POST request targeted at the scheduling Outbox
   collection of the calendar user requesting the information (the
   "Organizer").  To accomplish this, the request body MUST contain a
   "VFREEBUSY" calendar component with the "METHOD" iCalendar property
   set to the value "REQUEST" as specified in Section 3.3.2 of iTIP
   [RFC5546].  The resource identified by the Request-URI MUST be a
   resource collection of type CALDAV:schedule-outbox (Section 2.1).
   The "ORGANIZER" property value in the "VFREEBUSY" component MUST
   match one of the calendar user addresses of the owner of the Outbox
   collection.

   A response to a busy time request that indicates status for one or
   more calendar users MUST be an XML document with a CALDAV:schedule-
   response XML element as its root element.  This element MUST contain
   one CALDAV:response element for each calendar user, with each such
   element in turn containing elements that indicate which calendar user
   they correspond to, the scheduling status for that calendar user, any
   error codes, and an optional description.  For a successful busy time
   request, a CALDAV:calendar-data element is also present for each
   calendar user, containing the actual busy time information (i.e., an
   iCalendar "VFREEBUSY" component).  See Section 10 for details on the
   child elements.  See Appendix B.5 for an example busy time request
   and response.

5.1. Status Codes

   The list below summarizes the most common status codes used for this
   method.  However, clients need to be prepared to handle other
   2/3/4/5xx series status codes as well.

200 (OK) - The command succeeded.

204 (No Content) - The command succeeded.

400 (Bad Request) - The client has provided an invalid scheduling
message.

403 (Forbidden) - The client cannot submit a scheduling message to
the specified Request-URI.

404 (Not Found) - The URL in the Request-URI was not present.

      423 (Locked) - The specified resource is locked, and the client
      either is not a lock owner or the lock type requires a lock token
      to be submitted and the client did not submit it.

5.2. Additional Method Preconditions

   The following are existing preconditions that are reused for the POST
   method on an Outbox collection.

   o  DAV:need-privileges [RFC3744]

   o  CALDAV:supported-calendar-data [RFC4791]

   o  CALDAV:valid-calendar-data [RFC4791]

   o  CALDAV:max-resource-size [RFC4791]

   The following are new method preconditions for the POST method on an
   Outbox collection.

5.2.1. CALDAV:valid-scheduling-message Precondition

   Name:  valid-scheduling-message

   Namespace:  urn:ietf:params:xml:ns:caldav

   Apply to:  POST

   Use with:  400 Bad Request

   Purpose:  (precondition) -- The resource submitted in the POST
      request MUST obey all the restrictions specified in Section 3.3.2
      of iTIP [RFC5546].

   Definition:

     <!ELEMENT valid-scheduling-message EMPTY>

5.2.2. CALDAV:valid-organizer Precondition

   Name:  valid-organizer

   Namespace:  urn:ietf:params:xml:ns:caldav

   Apply to:  POST

   Use with:  403 Forbidden

   Purpose:  (precondition) -- The "ORGANIZER" property value in the
      POST request's scheduling message MUST match one of the calendar
      user addresses of the owner of the scheduling Outbox collection
      being targeted by the request.

   Definition:

     <!ELEMENT valid-organizer EMPTY>

6.  Scheduling Privileges

   New scheduling privileges are defined in this section.  All the
   scheduling privileges MUST be non-abstract and MUST appear in the
   DAV:supported-privilege-set property of scheduling Outbox and Inbox
   collections on which they are defined.

   The tables specified in Appendix A clarify which scheduling methods
   (e.g., "REQUEST", "REPLY", etc.) are controlled by each scheduling
   privilege defined in this section.

6.1. Privileges on Scheduling Inbox Collections

   This section defines new WebDAV Access Control List (ACL) [RFC3744]
   privileges that are defined for use on scheduling Inbox collections.
   These privileges determine whether delivery of scheduling messages
   from a calendar user is allowed by the calendar user who "owns" the
   scheduling Inbox collection.  This allows calendar users to choose
   which other calendar users can schedule with them.

   Note that when a scheduling message is delivered to a calendar user,
   in addition to a scheduling object resource being created in the
   calendar user's scheduling Inbox collection, a new scheduling object
   resource might be created or an existing one updated in a calendar
   belonging to the calendar user.  In that case, the ability to create
   or update the scheduling object resource in the calendar is
   controlled by the privileges assigned to the scheduling Inbox
   collection.

   The privileges defined in this section are ignored if applied to a
   resource other than a scheduling Inbox collection.

6.1.1. CALDAV:schedule-deliver Privilege

CALDAV:schedule-deliver is an aggregate privilege as per Section 6.3.

<!ELEMENT schedule-deliver EMPTY>

6.1.2. CALDAV:schedule-deliver-invite Privilege

   The CALDAV:schedule-deliver-invite privilege controls the processing
   and delivery of scheduling messages coming from an "Organizer".

     <!ELEMENT schedule-deliver-invite EMPTY>

6.1.3. CALDAV:schedule-deliver-reply Privilege

   The CALDAV:schedule-deliver-reply privilege controls the processing
   and delivery of scheduling messages coming from an "Attendee".

     <!ELEMENT schedule-deliver-reply EMPTY>

6.1.4. CALDAV:schedule-query-freebusy Privilege

   The CALDAV:schedule-query-freebusy privilege controls freebusy
   requests targeted at the owner of the scheduling Inbox collection.

     <!ELEMENT schedule-query-freebusy EMPTY>

6.2. Privileges on Scheduling Outbox Collections

   This section defines new WebDAV ACL [RFC3744] privileges that are
   defined for use on scheduling Outbox collections.  These privileges
   determine which calendar users are allowed to send scheduling
   messages on behalf of the calendar user who "owns" the scheduling
   Outbox collection.  This allows calendar users to choose other
   calendar users who can act on their behalf (e.g., assistants working
   on behalf of their boss).

The privileges defined in this section are ignored if applied to a
resource other than a scheduling Outbox collection.

6.2.1. CALDAV:schedule-send Privilege

CALDAV:schedule-send is an aggregate privilege as per Section 6.3.

<!ELEMENT schedule-send EMPTY>

6.2.2. CALDAV:schedule-send-invite Privilege

   The CALDAV:schedule-send-invite privilege controls the sending of
   scheduling messages by "Organizers".

   Users granted the DAV:bind privilege on a calendar collection, or the
   DAV:write privilege on scheduling object resources, will also need
   the CALDAV:schedule-send-invite privilege granted on the scheduling
   Outbox collection of the owner of the calendar collection or
   scheduling object resource in order to be allowed to create, modify,
   or delete scheduling object resources in a way that will trigger the
   CalDAV server to deliver scheduling messages to "Attendees".

     <!ELEMENT schedule-send-invite EMPTY>

6.2.3. CALDAV:schedule-send-reply Privilege

   The CALDAV:schedule-send-reply privilege controls the sending of
   scheduling messages by "Attendees".

   Users granted the DAV:bind privilege on a calendar collection, or the
   DAV:write privilege on scheduling object resources, will also need
   the CALDAV:schedule-send-reply privilege granted on the scheduling
   Outbox collection of the owner of the calendar collection or
   scheduling object resource in order to be allowed to create, modify,
   or delete scheduling object resources in a way that will trigger the
   CalDAV server to deliver scheduling message replies to the
   "Organizer".

     <!ELEMENT schedule-send-reply EMPTY>

6.2.4. CALDAV:schedule-send-freebusy Privilege

   The CALDAV:schedule-send-freebusy privilege controls the use of the
   POST method to submit scheduling messages that specify the scheduling
   method "REQUEST" with a "VFREEBUSY" calendar component.

     <!ELEMENT schedule-send-freebusy EMPTY>

6.3. Aggregation of Scheduling Privileges

   Server implementations MUST aggregate the scheduling privileges as
   follows:

      DAV:all contains CALDAV:schedule-deliver and CALDAV:schedule-send;

      CALDAV:schedule-deliver contains CALDAV:schedule-deliver-invite,
      CALDAV:schedule-deliver-reply, and CALDAV:schedule-query-freebusy;

      CALDAV:schedule-send contains CALDAV:schedule-send-invite, CALDAV:
      schedule-send-reply, and CALDAV:schedule-send-freebusy.

   The following diagram illustrates how scheduling privileges are
   aggregated according to the above requirements.

   [DAV:all] (aggregate)
       |
       +-- [CALDAV:schedule-deliver] (aggregate)
       |      |
       |      +-- [CALDAV:schedule-deliver-invite]
       |      +-- [CALDAV:schedule-deliver-reply]
       |      +-- [CALDAV:schedule-query-freebusy]
       |
       +-- [CALDAV:schedule-send] (aggregate)
              |
              +-- [CALDAV:schedule-send-invite]
              +-- [CALDAV:schedule-send-reply]
              +-- [CALDAV:schedule-send-freebusy]

7.  Additional iCalendar Property Parameters

   This specification defines additional iCalendar property parameters
   to support the CalDAV scheduling extensions.

7.1. Schedule Agent Parameter

   Parameter Name:  SCHEDULE-AGENT

   Purpose:  To specify the agent expected to deliver scheduling
      messages to the corresponding "Organizer" or "Attendee".

   Format Definition:  This property parameter is defined by the
      following notation:

     scheduleagentparam = "SCHEDULE-AGENT" "="
                      ("SERVER"      ; The server handles scheduling
                     / "CLIENT"      ; The client handles scheduling
                     / "NONE"        ; No scheduling
                     / x-name        ; Experimental type
                     / iana-token)   ; Other IANA-registered type
     ;
     ; If the parameter is not present, its value defaults to SERVER.
     ; "x-name" and "iana-token" are defined in Section 3.1 of
     ; [RFC5545].

   Description:  This property parameter MAY be specified on "ORGANIZER"
      or "ATTENDEE" iCalendar properties.  In the absence of this
      parameter, the value "SERVER" MUST be used for the default
      behavior.  The value determines whether or not a scheduling
      operation on a server will cause a scheduling message to be sent
      to the corresponding calendar user identified by the "ORGANIZER"
      or "ATTENDEE" property value.  When the value "SERVER" is
      specified, or the parameter is absent, then it is the server's
      responsibility to send a scheduling message as part of a
      scheduling operation.  When the value "CLIENT" is specified, that
      indicates that the client is handling scheduling messages with the
      calendar user itself.  When "NONE" is specified, no scheduling
      messages are being sent to the calendar user.

      Servers MUST NOT include this parameter in any scheduling messages
      sent as the result of a scheduling operation.

      Clients MUST NOT include this parameter in any scheduling messages
      that they themselves send.

      The parameter value MUST be the same on every "ORGANIZER" property
      in a scheduling object resource.

      The parameter value MUST be the same on each "ATTENDEE" property
      whose values match in a scheduling object resource.

      Servers and clients MUST treat x-name and iana-token values they
      do not recognize the same way as they would the "NONE" value.

   Example:

     ORGANIZER;SCHEDULE-AGENT=SERVER:mailto:bernard@example.com
     ATTENDEE;SCHEDULE-AGENT=NONE:mailto:cyrus@example.com

7.2. Schedule Force Send Parameter

   Parameter Name:  SCHEDULE-FORCE-SEND

   Purpose:  To force a scheduling message to be sent to the calendar
      user specified by the property.

   Format Definition:  This property parameter is defined by the
      following notation:

     scheduleforcesendparam = "SCHEDULE-FORCE-SEND" "="
                            ("REQUEST"    ; Force a "REQUEST"
                           / "REPLY"      ; Force a "REPLY"
                           / iana-token)
     ;
     ; "iana-token" is defined in Section 3.1 of [RFC5545].  Its value
     ; MUST be an IANA-registered iCalendar "METHOD" property value.

   Description:  This property parameter MAY be specified on "ATTENDEE"
      and "ORGANIZER" properties on which the "SCHEDULE-AGENT" property
      parameter is set to the value "SERVER" or is not specified.  This
      property parameter is used to force a server to send a scheduling
      message to a specific calendar user in situations where the server
      would not send a scheduling message otherwise (e.g., when no
      change that warrants the delivery of a new scheduling message was
      performed on the scheduling object resource).  An "Organizer" MAY
      specify this parameter on an "ATTENDEE" property with the value
      "REQUEST" to force a "REQUEST" scheduling message to be sent to
      this "Attendee".  An "Attendee" MAY specify this parameter on the
      "ORGANIZER" with the value "REPLY" to force a "REPLY" scheduling
      message to be sent to the "Organizer".

      Servers MUST NOT preserve this property parameter in scheduling
      object resources, nor include it in any scheduling messages sent
      as the result of a scheduling operation.

      Clients MUST NOT include this parameter in any scheduling messages
      that they themselves send.

      Servers MUST set the "SCHEDULE-STATUS" parameter of the "ATTENDEE"
      or "ORGANIZER" to 2.3 (i.e., "Success; invalid property parameter
      ignored"; see Section 3.6 of [RFC5546]) when the "SCHEDULE-FORCE-
      SEND" parameter is set to an iana-token value they do not
      recognize.

   Example:

     ORGANIZER;SCHEDULE-FORCE-SEND=REPLY:mailto:cyrus@example.com
     ATTENDEE;SCHEDULE-FORCE-SEND=REQUEST:mailto:bernard@example.com

7.3. Schedule Status Parameter

   Parameter Name:  SCHEDULE-STATUS

   Purpose:  To specify the status codes returned from processing of the
      most recent scheduling message sent to the corresponding
      "Attendee", or received from the corresponding "Organizer".

   Format Definition:  This property parameter is defined by the
      following notation:

     schedulestatusparam = "SCHEDULE-STATUS" "="
                         ( statcode
                        / DQUOTE statcode *("," statcode) DQUOTE)
     ;
     ; "statcode" is defined in Section 3.8.8.3 of [RFC5545].  The
     ; value is a single "statcode" or a comma-separated list of
     ; "statcode" values.

   Description:  This property parameter MAY be specified on the
      "ATTENDEE" and "ORGANIZER" properties.

      Servers MUST only add or change this property parameter on any
      "ATTENDEE" properties corresponding to calendar users who were
      sent a scheduling message via a scheduling operation.  Clients
      SHOULD NOT change or remove this parameter if it was provided by
      the server.  In the case where the client is handling the
      scheduling, the client MAY add, change, or remove this parameter
      to indicate the last scheduling message status it received.

      Servers MUST add this parameter to any "ORGANIZER" properties
      corresponding to calendar users who were sent a scheduling message
      reply by an "Attendee" via a scheduling operation.  Clients SHOULD
      NOT change or remove this parameter if it was provided by the
      server.  In the case where the client is handling the scheduling,
      the client MAY add, change, or remove this parameter to indicate
      the last scheduling message status it received.

      Servers MUST NOT include this parameter in any scheduling messages
      sent as the result of a scheduling operation.

      Clients MUST NOT include this parameter in any scheduling messages
      that they themselves send.

      Values for this property parameter are described in Section 3.2.9.

   Example:

     ATTENDEE;SCHEDULE-STATUS="2.0":mailto:bernard@example.com
     ATTENDEE;SCHEDULE-STATUS="2.0,2.4":mailto:cyrus@example.com

8.  Additional Message Header Fields

   This specification defines additional HTTP request and response
   headers for use with CalDAV.

8.1. Schedule-Reply Request Header

     Schedule-Reply = "Schedule-Reply" ":" ("T" | "F")

   Example:

     Schedule-Reply: F

   When an "Attendee" removes a scheduling object resource as per
   Section 3.2.2.4, and the Schedule-Reply header is set to the value
   "T" (true) or is not present, the server MUST send an appropriate
   reply scheduling message with the "Attendee's" "PARTSTAT" iCalendar
   property parameter value set to "DECLINED" as part of its normal
   scheduling operation processing.

   When the Schedule-Reply header is set to the value "F" (false), the
   server MUST NOT send a scheduling message as part of its normal
   scheduling operation processing.

   The Schedule-Reply request header is used by a client to indicate to
   a server whether or not a scheduling operation ought to occur when an
   "Attendee" deletes a scheduling object resource.  In particular, it
   controls whether a reply scheduling message is sent to the
   "Organizer" as a result of the removal.  There are situations in
   which unsolicited scheduling messages need to be silently removed (or
   ignored) for security or privacy reasons.  This request header allows
   the scheduling object resource to be removed if such a need arises.

8.2. Schedule-Tag Response Header

   The Schedule-Tag response header provides the current value of the
   CALDAV:schedule-tag property value.  The behavior of this response
   header is described in Section 3.2.10.

   All scheduling object resources MUST support the Schedule-Tag header.

     Schedule-Tag = "Schedule-Tag" ":" opaque-tag
     ; "opaque-tag" is defined in Section 3.11 of [RFC2616].

   Example:

     Schedule-Tag: "12ab34-cd56ef"

8.3. If-Schedule-Tag-Match Request Header

   The If-Schedule-Tag-Match request header field is used with a method
   to make it conditional.  Clients can set this header to the value
   returned in the Schedule-Tag response header, or the CALDAV:schedule-
   tag property, of a scheduling object resource previously retrieved
   from the server to avoid overwriting "consequential" changes to the
   scheduling object resource.

   All scheduling object resources MUST support the If-Schedule-Tag-
   Match header.

     If-Schedule-Tag-Match = "If-Schedule-Tag-Match" ":" opaque-tag
     ; "opaque-tag" is defined in Section 3.11 of [RFC2616].

   Example:

     If-Schedule-Tag-Match: "12ab34-cd56ef"

9.  Additional WebDAV Properties

   This specification defines the following new WebDAV properties for
   use with CalDAV.

9.1. CALDAV:schedule-calendar-transp Property

   Name:  schedule-calendar-transp

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Determines whether the calendar object resources in a
      calendar collection will affect the owner's busy time information.

   Protected:  This property MAY be protected and SHOULD NOT be returned
      by a PROPFIND DAV:allprop request (as defined in Section 14.2 of
      [RFC4918]).

   COPY/MOVE behavior:  This property value SHOULD be kept during a MOVE
      operation, and SHOULD be copied and preserved in a COPY.

Description:  This property SHOULD be defined on all calendar
      collections.  If present, it contains one of two XML elements that
      indicate whether the calendar object resources in the calendar
      collection ought to contribute to the owner's busy time.  When the
      CALDAV:opaque element is used, all calendar object resources in
      the corresponding calendar collection MUST contribute to busy
      time, assuming that access privileges and other iCalendar
      properties allow it to.  When the CALDAV:transparent XML element
      is used, the calendar object resources in the corresponding
      calendar collection MUST NOT contribute to busy time.

      If this property is not present on a calendar collection, then the
      default value CALDAV:opaque MUST be assumed.

   Definition:

     <!ELEMENT schedule-calendar-transp (opaque | transparent)>

     <!ELEMENT opaque EMPTY>
     

     <!ELEMENT transparent EMPTY>
     

   Example:

     <C:schedule-calendar-transp
          xmlns:C="urn:ietf:params:xml:ns:caldav">
       <C:opaque/>
     </C:schedule-calendar-transp>

9.2. CALDAV:schedule-default-calendar-URL Property

   Name:  schedule-default-calendar-URL

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Specifies a default calendar for an "Attendee" where new
      scheduling object resources are created.

   Protected:  This property MAY be protected in the case where a server
      does not support changing the default calendar, or does not
      support a default calendar.

   COPY/MOVE behavior:  This property is only defined on a scheduling
      Inbox collection that cannot be moved or copied.

   Description:  This property MAY be defined on a scheduling Inbox
      collection.  If present, it contains zero or one DAV:href XML
      elements.  When a DAV:href element is present, its value indicates
      a URL to a calendar collection that is used as the default
      calendar.  When no DAV:href element is present, it indicates that
      there is no default calendar.  In the absence of this property,
      there is no default calendar.  When there is no default calendar,
      the server is free to choose the calendar in which a new
      scheduling object resource is created.  See Section 4.3.

   Definition:

     <!ELEMENT schedule-default-calendar-URL (DAV:href?)>

   Example:

     <C:schedule-default-calendar-URL xmlns:D="DAV:"
         xmlns:C="urn:ietf:params:xml:ns:caldav">
       <D:href>/home/cyrus/calendars/work/</D:href>
     </C:schedule-default-calendar-URL>

9.3. CALDAV:schedule-tag Property

   Name:  schedule-tag

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Indicates whether a scheduling object resource has had a
      "consequential" change made to it.

   Value:  opaque-tag (defined in Section 3.11 of [RFC2616])

   Protected:  This property MUST be protected, as only the server can
      update the value.

   COPY/MOVE behavior:  This property value is determined by the server
      and MAY be different from the value on the source resource.

   Description:  The CALDAV:schedule-tag property MUST be defined on all
      scheduling object resources.  This property is described in
      Section 3.2.10.

   Definition:

     <!ELEMENT schedule-tag (#PCDATA)>

   Example:

     <C:schedule-tag xmlns:C="urn:ietf:params:xml:ns:caldav"
     >"12345-67890"</C:schedule-tag>

10. XML Element Definitions

10.1. CALDAV:schedule-response XML Element

   Name:  schedule-response

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Contains the set of responses for a POST method request.

   Description:  See Section 5.

   Definition:

     <!ELEMENT schedule-response (response*)>

10.2. CALDAV:response XML Element

Name:  response

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Contains a single response for a POST method request.

   Description:  See Section 5.

   Definition:

     <!ELEMENT response (recipient,
                         request-status,
                         calendar-data?,
                         DAV:error?,
                         DAV:responsedescription?)>

10.3. CALDAV:recipient XML Element

Name: recipient

Namespace: urn:ietf:params:xml:ns:caldav

   Purpose:  The calendar user address that the enclosing response for a
      POST method request is for.

   Description:  See Section 5.

   Definition:

     <!ELEMENT recipient (DAV:href)>

10.4. CALDAV:request-status XML Element

   Name:  request-status

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  The iTIP "REQUEST-STATUS" property value for this response.

   Description:  See Section 5.

   Definition:

     <!ELEMENT request-status (#PCDATA)>

11.  Security Considerations

   The process of scheduling involves the sending and receiving of
   scheduling messages.  As a result, the security problems related to
   messaging in general are relevant here.  In particular, the
   authenticity of the scheduling messages needs to be verified.
   Servers and clients MUST use an HTTP connection protected with
   Transport Layer Security (TLS) as defined in [RFC2818] for all
   scheduling operations.  Clients MUST use the procedures detailed in
   Section 6 of [RFC6125] to verify the authenticity of the server.
   Servers MUST make use of HTTP authentication [RFC2617] to verify the
   authenticity of the calendar user for whom the client is sending
   requests.

11.1. Preventing Denial-of-Service Attacks

   Servers MUST ensure that clients cannot consume excessive server
   resources by carrying out "large" scheduling operations.  In
   particular, servers SHOULD enforce CALDAV:max-resource-size, CALDAV:
   max-instances, and CALDAV:max-attendees-per-instance preconditions as
   applicable for scheduling Inbox and Outbox collections.

11.2. Verifying Scheduling Operations

   When handling a scheduling operation:

   1.  Servers MUST verify that the principal associated with the DAV:
       owner of the calendar collection in which a scheduling object
       resource is being manipulated contains a CALDAV:schedule-outbox-
       URL property value.

   2.  Servers MUST verify that the currently authenticated user has the
       CALDAV:schedule-send privilege, or a sub-privilege aggregated
       under this privilege, on the scheduling Outbox collection of the
       DAV:owner of the calendar collection in which a scheduling object
       resource is being manipulated.

   3.  Servers MUST only deliver scheduling messages to recipients when
       the CALDAV:schedule-deliver privilege, or a sub-privilege
       aggregated under this privilege, is granted on the recipient's
       scheduling Inbox collection for the principal associated with the
       DAV:owner of the calendar collection in which a scheduling object
       resource is being manipulated.

   4.  To prevent impersonation of calendar users, the server MUST
       verify that the "ORGANIZER" property in an organizer scheduling
       object resource matches one of the calendar user addresses of the
       DAV:owner of the calendar collection in which the resource is
       stored.

   5.  To prevent spoofing of an existing scheduling object resource,
       servers MUST verify that the "UID" iCalendar property value in a
       new scheduling object resource does not match that of an existing
       scheduling object resource with a different "ORGANIZER" property
       value.

11.3. Verifying Busy Time Information Requests

   When handling a POST request on a scheduling Outbox collection:

   1.  Servers MUST verify that the principal associated with the
       calendar user address specified in the "ORGANIZER" property of
       the scheduling message data in the request contains a CALDAV:
       schedule-outbox-URL property value that matches the scheduling
       Outbox collection targeted by the request.

   2.  Servers MUST verify that the currently authenticated user has the
       CALDAV:schedule-send privilege, or a sub-privilege aggregated
       under this privilege, on the scheduling Outbox collection
       targeted by the request.

   3.  Servers MUST only return valid freebusy information for
       recipients when the CALDAV:schedule-deliver privilege, or a
       sub-privilege aggregated under this privilege, is granted on the
       recipient's scheduling Inbox collection for the principal
       associated with the DAV:owner of the scheduling Outbox collection
       targeted by the request.

11.4. Privacy Issues

   This specification only defines how calendar users on the same server
   are able to schedule with each other -- unauthenticated users have no
   way to carry out scheduling operations.  Access control privileges
   (as per Section 6) can control which of those users can schedule with
   others.  Calendar users not wishing to expose their calendar
   information to other users can do so by denying privileges to
   specific users, or all users, for all scheduling operations, or
   perhaps only freebusy.

   "Attendees" can also use the Schedule-Reply request header
   (Section 8.1) with the value set to "F" to prevent notification to an
   "Organizer" that a scheduling object resource was deleted.  This
   allows "Attendees" to remove unwanted scheduling messages without any
   response to the "Organizer".

   Servers MUST NOT expose any private iCalendar data, or WebDAV
   resource state information (URLs, WebDAV properties, etc.) for one
   calendar user to another via scheduling messages or error responses
   to scheduling operations.  In particular, as per Section 8.1 of
   [RFC4918], authorization errors MUST take preference over other
   errors.

11.5. Mitigation of iTIP Threats

   Section 6.1 of iTIP [RFC5546] defines a set of potential threats in a
   scheduling system, and Section 6.2 of [RFC5546] defines
   recommendations on how those can be addressed in protocols using
   iTIP.  This specification addresses the iTIP threats in the following
   manner:

   Spoofing the "Organizer":  Addressed by item 4 in Section 11.2.

   Spoofing the "Attendee":  Addressed by Section 3.2.2.1 and item 2 in
      Section 11.2.

   Unauthorized Replacement of the "Organizer":  Addressed by item 5 in
      Section 11.2.

   Eavesdropping and Data Integrity:  Addressed by requiring TLS.

   Flooding a Calendar:  Addressed by requirements in Section 11.1.

   Unauthorized REFRESH Requests:  This specification does not support
      the REFRESH method.

12. IANA Considerations

12.1. Message Header Field Registrations

The message header fields below have been added to the Permanent
Message Header Field Registry (see [RFC3864]).

12.1.1. Schedule-Reply

   Header field name: Schedule-Reply

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document(s): this specification (Section 8.1)

   Related information: none

12.1.2. Schedule-Tag

   Header field name: Schedule-Tag

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document(s): this specification (Section 8.2)

   Related information: none

12.1.3. If-Schedule-Tag-Match

   Header field name: If-Schedule-Tag-Match

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document(s): this specification (Section 8.3)

   Related information: none

12.2. iCalendar Property Parameter Registrations

12.3. iCalendar REQUEST-STATUS Value Registrations

12.4. Additional iCalendar Elements Registries

   Per this specification, two new IANA registries for iCalendar
   elements have been added.  Additional codes MAY be used, provided the
   process described in Section 8.2.1 of [RFC5545] is used to register
   them.

12.4.1. Schedule Agent Values Registry

12.4.2. Schedule Force Send Values Registry

13. Acknowledgements

   The authors would like to thank the following individuals for
   contributing their ideas and support for writing this specification:
   Mike Douglass, Lisa Dusseault, Red Dutta, Jacob Farkas, Jeffrey
   Harris, Helge Hess, Eliot Lear, Andrew McMillan, Alexey Melnikov,
   Arnaud Quillaud, Julian F. Reschke, Wilfredo Sanchez Vega, and Simon
   Vaillancourt.

   The authors would also like to thank the Calendaring and Scheduling
   Consortium for advice with this specification, and for organizing
   interoperability testing events to help refine it.

14. References

14.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3744]  Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, "Web
              Distributed Authoring and Versioning (WebDAV)
              Access Control Protocol", RFC 3744, May 2004.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              March 2007.

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234,
              January 2008.

   [RFC5545]  Desruisseaux, B., Ed., "Internet Calendaring and
              Scheduling Core Object Specification (iCalendar)",
              RFC 5545, September 2009.

   [RFC5546]  Daboo, C., Ed., "iCalendar Transport-Independent
              Interoperability Protocol (iTIP)", RFC 5546,
              December 2009.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
              and F. Yergeau, "Extensible Markup Language (XML) 1.0
              (Fifth Edition)", World Wide Web Consortium
              Recommendation REC-xml-20081126, November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126>.

14.2. Informative References

   [RFC6047]  Melnikov, A., Ed., "iCalendar Message-Based
              Interoperability Protocol (iMIP)", RFC 6047,
              December 2010.

Appendix A. Scheduling Privileges Summary

A.1. Scheduling Inbox Privileges

   The following tables specify which scheduling privileges grant the
   right to a calendar user to deliver a scheduling message to the
   scheduling Inbox collection of another calendar user.  The
   appropriate behavior depends on the calendar component type as well
   as the scheduling "METHOD" specified in the scheduling message.

                                 +--------------------------------+
                                 |  METHOD for VEVENT and VTODO   |
   +-----------------------------+---------+-------+-----+--------+
   | Scheduling Inbox Privilege  | REQUEST | REPLY | ADD | CANCEL |
   +-----------------------------+---------+-------+-----+--------+
   | schedule-deliver            |    *    |   *   |  *  |   *    |
   |   schedule-deliver-invite   |    *    |       |  *  |   *    |
   |   schedule-deliver-reply    |         |   *   |     |        |
   |   schedule-query-freebusy   |         |       |     |        |
   +-----------------------------+---------+-------+-----+--------+

                                 +----------------------+
                                 | METHOD for VFREEBUSY |
   +-----------------------------+----------------------+
   | Scheduling Inbox Privilege  |       REQUEST        |
   +-----------------------------+----------------------+
   | schedule-deliver            |          *           |
   |   schedule-deliver-invite   |                      |
   |   schedule-deliver-reply    |                      |
   |   schedule-query-freebusy   |          *           |
   +-----------------------------+----------------------+

A.2. Scheduling Outbox Privileges

   The following tables specify which scheduling privileges grant the
   right to a calendar user to perform busy time information requests
   and to submit scheduling messages to other calendar users as the
   result of a scheduling operation.  The appropriate behavior depends
   on the calendar component type as well as the scheduling "METHOD"
   specified in the scheduling message.

                                 +--------------------------------+
                                 |  METHOD for VEVENT and VTODO   |
   +-----------------------------+---------+-------+-----+--------+
   | Scheduling Outbox Privilege | REQUEST | REPLY | ADD | CANCEL |
   +-----------------------------+---------+-------+-----+--------+
   | schedule-send               |    *    |   *   |  *  |   *    |
   |   schedule-send-invite      |    *    |       |  *  |   *    |
   |   schedule-send-reply       |         |   *   |     |        |
   |   schedule-send-freebusy    |         |       |     |        |
   +-----------------------------+---------+-------+-----+--------+

                                 +----------------------+
                                 | METHOD for VFREEBUSY |
   +-----------------------------+----------------------+
   | Scheduling Outbox Privilege |       REQUEST        |
   +-----------------------------+----------------------+
   | schedule-send               |          *           |
   |   schedule-send-invite      |                      |
   |   schedule-send-reply       |                      |
   |   schedule-send-freebusy    |          *           |
   +-----------------------------+----------------------+

Appendix B. Example Scheduling Operations

   This section describes some example scheduling operations that give a
   general idea of how scheduling is carried out between CalDAV clients
   and servers from the perspective of meeting "Organizers" and
   "Attendees".

   The server is assumed to be hosted in the "example.com" domain, and
   users whose email addresses are at the "example.com" domain are
   assumed to be hosted by the server.  In addition, the email addresses
   in the "example.net" domain are also valid email addresses for
   calendar users hosted by the server.  Calendar users with an email
   address at the "example.org" domain are assumed to not be hosted by
   the server.

   In the following examples, the requests and responses are incomplete
   and are only for illustrative purposes.  In particular, HTTP
   authentication headers and behaviors are not shown, even though they
   are required in normal operation.

B.1. Example: "Organizer" Inviting Multiple "Attendees"

   In the following example, Cyrus invites Wilfredo, Bernard, and Mike
   to a single instance event by simply creating a new scheduling object
   resource in one of his calendar collections by using the PUT method.

   >> Request <<

   PUT /home/cyrus/calendars/work/9263504FD3AD.ics HTTP/1.1
   Host: cal.example.com
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx
   If-None-Match: *

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Client//EN
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090602T185254Z
   DTSTART:20090602T160000Z
   DTEND:20090602T170000Z
   TRANSP:OPAQUE
   SUMMARY:Lunch
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com
   ATTENDEE;CN="Wilfredo Sanchez Vega";CUTYPE=INDIVIDUAL;PARTSTAT
    =NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:wilfredo@
    example.com
   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:bernard@ex
    ample.net
   ATTENDEE;CN="Mike Douglass";CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-A
    CTION;RSVP=TRUE:mailto:mike@example.org
   END:VEVENT
   END:VCALENDAR

   >> Response <<

   HTTP/1.1 201 Created
   Content-Length: 0

   Date: Tue, 02 Jun 2009 18:52:54 GMT
   Last-Modified: Tue, 02 Jun 2009 18:52:54 GMT
   ETag: "d85561cfe74a4e785eb4639451b434fb"
   Schedule-Tag: "488177c8-2ea7-4176-a6cb-fab8cfccdea2"

   Once the event creation has been completed, Cyrus's client will
   retrieve the event back from the server to get the schedule status of
   each "Attendee", as well as record the Schedule-Tag value for future
   use.  In this example, the server reports that a scheduling message
   was delivered to Wilfredo, a scheduling message is still pending for
   Bernard, and the server was unable to deliver a scheduling message to
   Mike.

   >> Request <<

   GET /home/cyrus/calendars/work/9263504FD3AD.ics HTTP/1.1
   Host: cal.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Tue, 02 Jun 2009 18:52:58 GMT
   Last-Modified: Tue, 02 Jun 2009 18:52:58 GMT
   ETag: "eb897deabc8939589da116714bc99265"
   Schedule-Tag: "488177c8-2ea7-4176-a6cb-fab8cfccdea2"
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090602T185300Z
   DTSTART:20090602T160000Z
   DTEND:20090602T170000Z
   TRANSP:OPAQUE
   SUMMARY:Lunch
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com
   ATTENDEE;CN="Wilfredo Sanchez Vega";CUTYPE=INDIVIDUAL;PARTSTAT
    =NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE;SCHEDULE-STATUS=
    1.2:mailto:wilfredo@e
    xample.com

   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE;SCHEDULE-STATUS=
    1.0:mailto:bernard@example.net
   ATTENDEE;CN="Mike Douglass";CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-A
    CTION;RSVP=TRUE;SCHEDULE-STATUS=3.7:mailto:mike@example.org
   END:VEVENT
   END:VCALENDAR

B.2. Example: "Attendee" Receiving an Invitation

   In the following example, Wilfredo's client retrieves and deletes the
   new scheduling message that appeared in his scheduling Inbox
   collection after the server automatically processed it and created a
   new scheduling object resource in his default calendar collection.

   >> Request <<

   GET /home/wilfredo/calendars/inbox/27d93fc0a58c.ics HTTP/1.1
   Host: cal.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Tue, 02 Jun 2009 18:59:58 GMT
   Last-Modified: Tue, 02 Jun 2009 18:59:58 GMT
   ETag: "da116714bc9926c89395895eb897deab"
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   METHOD:REQUEST
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090602T185254Z
   DTSTART:20090602T160000Z
   DTEND:20090602T170000Z
   TRANSP:OPAQUE
   SUMMARY:Lunch
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com
   ATTENDEE;CN="Wilfredo Sanchez Vega";CUTYPE=INDIVIDUAL;PARTSTAT
    =NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:wilfredo@
    example.com

   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:bernard@ex
    ample.net
   ATTENDEE;CN="Mike Douglass";CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-A
    CTION;RSVP=TRUE:mailto:mike@example.org
   END:VEVENT
   END:VCALENDAR

   >> Request <<

   DELETE /home/wilfredo/calendars/inbox/27d93fc0a58c.ics HTTP/1.1
   Host: cal.example.com

   >> Response <<

   HTTP/1.1 204 No Content
   Date: Tue, 02 Jun 2009 20:40:36 GMT

B.3. Example: "Attendee" Replying to an Invitation

   In the following example, Wilfredo accepts Cyrus's invitation and
   sets an alarm reminder on the event.  It uses the If-Schedule-Tag-
   Match precondition behavior to ensure it does not overwrite any
   significant changes from the "Organizer" that might have occurred
   after it retrieved the initial resource data.

   >> Request <<

   PUT /home/wilfredo/calendars/work/BB64861C2228.ics HTTP/1.1
   Host: cal.example.com
   If-Schedule-Tag-Match: "e78f23ed-0188-4bab-938d-2aeb3324c7e8"
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Client//EN
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090602T185254Z
   DTSTART:20090602T160000Z
   DTEND:20090602T170000Z
   TRANSP:OPAQUE
   SUMMARY:Lunch
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com

   ATTENDEE;CN="Wilfredo Sanchez Vega";CUTYPE=INDIVIDUAL;PARTSTAT
    =ACCEPTED;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:wilfredo@exam
    ple.com
   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:bernard@ex
    ample.net
   ATTENDEE;CN="Mike Douglass";CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-A
    CTION;RSVP=TRUE:mailto:mike@example.org
   BEGIN:VALARM
   TRIGGER:-PT15M
   ACTION:DISPLAY
   DESCRIPTION:Reminder
   END:VALARM
   END:VEVENT
   END:VCALENDAR

   >> Response <<

   HTTP/1.1 200 OK
   Content-Length: 0
   Date: Tue, 02 Jun 2009 18:57:54 GMT
   Last-Modified: Tue, 02 Jun 2009 18:57:54 GMT
   ETag: "eb4639451b434fbd85561cfe74a4e785"
   Schedule-Tag: "8893ee45-eb9d-428f-b53c-c777daf19e41"

   Once the event modification has been completed, Wilfredo's client
   will retrieve the event back from the server to get the schedule
   status of the "Organizer".

   >> Request <<

   GET /home/wilfredo/calendars/work/BB64861C2228.ics HTTP/1.1
   Host: cal.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Tue, 02 Jun 2009 19:03:03 GMT
   Last-Modified: Tue, 02 Jun 2009 19:02:21 GMT
   ETag: "5eb897deabda116714bc9926c8939589"
   Schedule-Tag: "8893ee45-eb9d-428f-b53c-c777daf19e41"
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Client//EN
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090602T190221Z
   DTSTART:20090602T160000Z
   DTEND:20090602T170000Z
   TRANSP:OPAQUE
   SUMMARY:Lunch
   ORGANIZER;CN="Cyrus Daboo";SCHEDULE-STATUS=1.2:mailto:cyrus@ex
    ample.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com
   ATTENDEE;CN="Wilfredo Sanchez Vega";CUTYPE=INDIVIDUAL;PARTSTAT
    =ACCEPTED;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:wilfredo@exam
    ple.com
   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:bernard@ex
    ample.net
   ATTENDEE;CN="Mike Douglass";CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-A
    CTION;RSVP=TRUE:mailto:mike@example.org
   BEGIN:VALARM
   TRIGGER:-PT15M
   ACTION:DISPLAY
   DESCRIPTION:Reminder
   END:VALARM
   END:VEVENT
   END:VCALENDAR

B.4. Example: "Organizer" Receiving a Reply to an Invitation

   On reception of Wilfredo's reply, Cyrus's server will automatically
   update Cyrus's scheduling object resource, make Wilfredo's scheduling
   message available in Cyrus's scheduling Inbox collection, and deliver
   an updated scheduling message to Bernard to share Wilfredo's updated
   participation status.  In this example, Cyrus's client retrieves and
   deletes this scheduling message in his scheduling Inbox collection.

   >> Request <<

   GET /home/cyrus/calendars/inbox/c0a58c27d93f.ics HTTP/1.1
   Host: cal.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Tue, 02 Jun 2009 19:05:02 GMT
   Last-Modified: Tue, 02 Jun 2009 19:04:20 GMT
   ETag: "9265eb897deabc8939589da116714bc9"
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   METHOD:REPLY
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090602T185754Z
   DTSTART:20090602T160000Z
   DTEND:20090602T170000Z
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Wilfredo Sanchez Vega";PARTSTAT=ACCEPTED:mailto:w
    ilfredo@example.com
   REQUEST-STATUS:2.0;Success
   END:VEVENT
   END:VCALENDAR

   >> Request <<

   DELETE /home/cyrus/calendars/inbox/c0a58c27d93f.ics HTTP/1.1
   Host: cal.example.com

   >> Response <<

   HTTP/1.1 204 No Content
   Date: Tue, 02 Jun 2009 19:05:05 GMT

   Cyrus's client then retrieves the event back from the server with
   Wilfredo's updated participation status.

   >> Request <<

   GET /home/cyrus/calendars/work/9263504FD3AD.ics HTTP/1.1
   Host: cal.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Tue, 02 Jun 2009 19:05:02 GMT
   Last-Modified: Tue, 02 Jun 2009 19:04:20 GMT
   ETag: "eb897deabc8939589da116714bc99265"
   Schedule-Tag: "132cab27-1fe3-67ab-de13-abd348d1dee3"
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090602T190420Z
   DTSTART:20090602T160000Z
   DTEND:20090602T170000Z
   TRANSP:OPAQUE
   SUMMARY:Lunch
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com
   ATTENDEE;CN="Wilfredo Sanchez Vega";CUTYPE=INDIVIDUAL;PARTSTAT
    =ACCEPTED;ROLE=REQ-PARTICIPANT;RSVP=TRUE;SCHEDULE-STATUS=2.0:
    mailto:wilfredo@example.com
   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE;SCHEDULE-STATUS=1
    .0:mailto:bernard@example.net
   ATTENDEE;CN="Mike Douglass";CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-A
    CTION;RSVP=TRUE;SCHEDULE-STATUS=3.7:mailto:mike@example.org
   END:VEVENT
   END:VCALENDAR

B.5. Example: "Organizer" Requesting Busy Time Information

   In this example, Cyrus requests the busy time information of
   Wilfredo, Bernard, and Mike.

   >> Request <<

   POST /home/cyrus/calendars/outbox/ HTTP/1.1
   Host: cal.example.com
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Client//EN
   METHOD:REQUEST
   BEGIN:VFREEBUSY
   UID:4FD3AD926350
   DTSTAMP:20090602T190420Z
   DTSTART:20090602T000000Z
   DTEND:20090604T000000Z
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Wilfredo Sanchez Vega":mailto:wilfredo@example.com
   ATTENDEE;CN="Bernard Desruisseaux":mailto:bernard@example.net
   ATTENDEE;CN="Mike Douglass":mailto:mike@example.org
   END:VFREEBUSY
   END:VCALENDAR

   >> Response <<

   HTTP/1.1 200 OK
   Date: Tue, 02 Jun 2009 20:07:34 GMT
   Content-Type: application/xml; charset="utf-8"
   Content-Length: xxxx

   <?xml version="1.0" encoding="utf-8" ?>
   <C:schedule-response xmlns:D="DAV:"
          xmlns:C="urn:ietf:params:xml:ns:caldav">
   <C:response>
   <C:recipient>
   <D:href>mailto:wilfredo@example.com</D:href>
   </C:recipient>
   <C:request-status>2.0;Success</C:request-status>
   <C:calendar-data>BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   METHOD:REPLY
   BEGIN:VFREEBUSY

   UID:4FD3AD926350
   DTSTAMP:20090602T200733Z
   DTSTART:20090602T000000Z
   DTEND:20090604T000000Z
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Wilfredo Sanchez Vega":mailto:wilfredo@example.com
   FREEBUSY;FBTYPE=BUSY:20090602T110000Z/20090602T120000Z
   FREEBUSY;FBTYPE=BUSY:20090603T170000Z/20090603T180000Z
   END:VFREEBUSY
   END:VCALENDAR
   </C:calendar-data>
   </C:response>
   <C:response>
   <C:recipient>
   <D:href>mailto:bernard@example.net</D:href>
   </C:recipient>
   <C:request-status>2.0;Success</C:request-status>
   <C:calendar-data>BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   METHOD:REPLY
   BEGIN:VFREEBUSY
   UID:4FD3AD926350
   DTSTAMP:20090602T200733Z
   DTSTART:20090602T000000Z
   DTEND:20090604T000000Z
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Bernard Desruisseaux":mailto:bernard@example.net
   FREEBUSY;FBTYPE=BUSY:20090602T150000Z/20090602T160000Z
   FREEBUSY;FBTYPE=BUSY:20090603T090000Z/20090603T100000Z
   FREEBUSY;FBTYPE=BUSY:20090603T180000Z/20090603T190000Z
   END:VFREEBUSY
   END:VCALENDAR
   </C:calendar-data>
   </C:response>
   <C:response>
   <C:recipient>
   <D:href>mailto:mike@example.org</D:href>
   </C:recipient>
   <C:request-status>3.7;Invalid calendar user</C:request-status>
   </C:response>
   </C:schedule-response>

B.6. Example: User Attempting to Invite "Attendee" on Behalf of
"Organizer"

   In the following example, Cyrus attempts to create, on behalf of
   Wilfredo, an event with Bernard specified as an "Attendee".  The
   request fails, since Wilfredo didn't grant Cyrus the right to invite
   other calendar users on his behalf.

   >> Request <<

   PUT /home/wilfredo/calendars/work/def456.ics HTTP/1.1
   Host: cal.example.com
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx
   If-None-Match: *

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Client//EN
   BEGIN:VEVENT
   UID:3504F926D3AD
   SEQUENCE:0
   DTSTAMP:20090602T190221Z
   DTSTART:20090602T230000Z
   DTEND:20090603T000000Z
   TRANSP:OPAQUE
   SUMMARY:Dinner
   ORGANIZER;CN="Wilfredo Sanchez Vega":mailto:wilfredo@example.com
   ATTENDEE;CN="Wilfredo Sanchez Vega";CUTYPE=INDIVIDUAL;PARTSTAT=A
    CCEPTED:mailto:wilfredo@example.com
   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=NE
    EDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:bernard@exampl
    e.net
   END:VEVENT
   END:VCALENDAR

   >> Response <<

   HTTP/1.1 403 Forbidden
   Content-Type: application/xml; charset="utf-8"
   Content-Length: xxxx

   <?xml version="1.0" encoding="utf-8" ?>
   <D:error xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
     <D:need-privileges>
       <D:resource>
         <D:href>/home/wilfredo/calendars/outbox/</D:href>
         <D:privilege><C:schedule-send-invite/></D:privilege>
       </D:resource>
     </D:need-privileges>
   </D:error>

B.7. Example: "Attendee" Declining an Instance of a Recurring Event

   In the following example, Bernard declines the second recurrence
   instance of a daily recurring event he's been invited to by Cyrus.

   >> Request <<

   PUT /home/bernard/calendars/work/4FD3AD926350.ics HTTP/1.1
   Host: cal.example.com
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx
   If-Schedule-Tag-Match: "7775FB30-7534-489E-A79A-0EA147B933EB"

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Client//EN
   BEGIN:VTIMEZONE
   TZID:America/Montreal
   BEGIN:STANDARD
   DTSTART:20071104T020000
   RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
   TZNAME:EST
   TZOFFSETFROM:-0400
   TZOFFSETTO:-0500
   END:STANDARD
   BEGIN:DAYLIGHT
   DTSTART:20070311T020000
   RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
   TZNAME:EDT
   TZOFFSETFROM:-0500
   TZOFFSETTO:-0400
   END:DAYLIGHT

   END:VTIMEZONE
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090602T185254Z
   DTSTART;TZID=America/Montreal:20090601T150000
   DTEND;TZID=America/Montreal:20090601T160000
   RRULE:FREQ=DAILY;INTERVAL=1;COUNT=5
   TRANSP:OPAQUE
   SUMMARY:Review Internet-Draft
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com
   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    ACCEPTED;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:bernard@exampl
    e.net
   END:VEVENT
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090603T183823Z
   RECURRENCE-ID;TZID=America/Montreal:20090602T150000
   DTSTART;TZID=America/Montreal:20090602T150000
   DTEND;TZID=America/Montreal:20090602T160000
   TRANSP:TRANSPARENT
   SUMMARY:Review Internet-Draft
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com
   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    DECLINED;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:bernard@exampl
    e.net
   END:VEVENT
   END:VCALENDAR

   >> Response <<

   HTTP/1.1 200 OK
   Content-Length: 0
   Date: Tue, 02 Jun 2009 18:52:54 GMT
   Last-Modified: Tue, 02 Jun 2009 18:52:54 GMT
   ETag: "d85561cfe74a4e785eb4639451b434fb"
   Schedule-Tag: "488177c8-2ea7-4176-a6cb-fab8cfccdea2"

   Bernard's participation status update will cause his server to
   deliver a scheduling message to Cyrus.  Cyrus's client will find the
   following reply message from Bernard in Cyrus's scheduling Inbox
   collection:

   >> Request <<

   GET /home/cyrus/calendars/inbox/9263504FD3AD.ics HTTP/1.1
   Host: cal.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Tue, 02 Jun 2009 18:52:58 GMT
   Last-Modified: Tue, 02 Jun 2009 18:52:58 GMT
   ETag: "eb897deabc8939589da116714bc99265"
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Client//EN
   METHOD:REPLY
   BEGIN:VTIMEZONE
   TZID:America/Montreal
   BEGIN:STANDARD
   DTSTART:20071104T020000
   RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
   TZNAME:EST
   TZOFFSETFROM:-0400
   TZOFFSETTO:-0500
   END:STANDARD
   BEGIN:DAYLIGHT
   DTSTART:20070311T020000
   RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
   TZNAME:EDT
   TZOFFSETFROM:-0500
   TZOFFSETTO:-0400
   END:DAYLIGHT
   END:VTIMEZONE
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090603T183823Z
   RECURRENCE-ID;TZID=America/Montreal:20090602T150000
   DTSTART;TZID=America/Montreal:20090602T150000
   DTEND;TZID=America/Montreal:20090602T160000
   SUMMARY:Review Internet-Draft

   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Bernard Desruisseaux";PARTSTAT=DECLINED:
    mailto:bernard@example.net
   REQUEST-STATUS:2.0;Success
   END:VEVENT
   END:VCALENDAR

B.8. Example: "Attendee" Removing an Instance of a Recurring Event

   In the following example, Bernard removes from his calendar the third
   recurrence instance of a daily recurring event he's been invited to
   by Cyrus.  This is accomplished by the addition of an "EXDATE"
   property to the scheduling object resource stored by Bernard.

   >> Request <<

   PUT /home/bernard/calendars/work/4FD3AD926350.ics HTTP/1.1
   Host: cal.example.com
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx
   If-Schedule-Tag-Match: "488177c8-2ea7-4176-a6cb-fab8cfccdea2"

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Client//EN
   BEGIN:VTIMEZONE
   TZID:America/Montreal
   BEGIN:STANDARD
   DTSTART:20071104T020000
   RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
   TZNAME:EST
   TZOFFSETFROM:-0400
   TZOFFSETTO:-0500
   END:STANDARD
   BEGIN:DAYLIGHT
   DTSTART:20070311T020000
   RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
   TZNAME:EDT
   TZOFFSETFROM:-0500
   TZOFFSETTO:-0400
   END:DAYLIGHT
   END:VTIMEZONE
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090602T185254Z
   DTSTART;TZID=America/Montreal:20090601T150000
   DTEND;TZID=America/Montreal:20090601T160000

   RRULE:FREQ=DAILY;INTERVAL=1;COUNT=5
   EXDATE;TZID=America/Montreal:20090603T150000
   TRANSP:OPAQUE
   SUMMARY:Review Internet-Draft
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com
   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    ACCEPTED;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:bernard@exampl
    e.net
   END:VEVENT
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090603T183823Z
   RECURRENCE-ID;TZID=America/Montreal:20090602T150000
   DTSTART;TZID=America/Montreal:20090602T150000
   DTEND;TZID=America/Montreal:20090602T160000
   TRANSP:TRANSPARENT
   SUMMARY:Review Internet-Draft
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Cyrus Daboo";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:
    mailto:cyrus@example.com
   ATTENDEE;CN="Bernard Desruisseaux";CUTYPE=INDIVIDUAL;PARTSTAT=
    DECLINED;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:bernard@exampl
    e.net
   END:VEVENT
   END:VCALENDAR

   Bernard's deletion of a recurrence instance will cause his server to
   deliver a scheduling message to Cyrus.  Cyrus's client will find the
   following reply message from Bernard in Cyrus's scheduling Inbox
   collection:

   >> Request <<

   GET /home/cyrus/calendars/inbox/6504923FD3AD.ics HTTP/1.1
   Host: cal.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Tue, 02 Jun 2009 18:52:58 GMT
   Last-Modified: Tue, 02 Jun 2009 18:52:58 GMT
   ETag: "eb897deabc8939589da116714bc99265"
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Client//EN
   METHOD:REPLY
   BEGIN:VTIMEZONE
   TZID:America/Montreal
   BEGIN:STANDARD
   DTSTART:20071104T020000
   RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
   TZNAME:EST
   TZOFFSETFROM:-0400
   TZOFFSETTO:-0500
   END:STANDARD
   BEGIN:DAYLIGHT
   DTSTART:20070311T020000
   RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
   TZNAME:EDT
   TZOFFSETFROM:-0500
   TZOFFSETTO:-0400
   END:DAYLIGHT
   END:VTIMEZONE
   BEGIN:VEVENT
   UID:9263504FD3AD
   SEQUENCE:0
   DTSTAMP:20090603T183823Z
   RECURRENCE-ID;TZID=America/Montreal:20090603T150000
   DTSTART;TZID=America/Montreal:20090603T150000
   DTEND;TZID=America/Montreal:20090603T160000
   SUMMARY:Review Internet-Draft
   ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com
   ATTENDEE;CN="Bernard Desruisseaux";PARTSTAT=DECLINED:
    mailto:bernard@example.net
   REQUEST-STATUS:2.0;Success
   END:VEVENT
   END:VCALENDAR

Authors' Addresses

   Cyrus Daboo
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   EMail: cyrus@daboo.name
   URI:   http://www.apple.com/

   Bernard Desruisseaux
   Oracle Corporation
   600 Blvd. de Maisonneuve West
   Suite 1900
   Montreal, QC  H3A 3J2
   CANADA

   EMail: bernard.desruisseaux@oracle.com
   URI:   http://www.oracle.com/